Maybe you have seen them out there, at least a hundred (yes, I mean a hundred) web-sites condemning the Watchtower Society for using Johannes Greber's New Testament to support their rendering of "a god" at John 1:1c. Most Jehovah's Witnesses do not even know who Johannes Greber is, and the opposers that DO mention him know embarrassingly little of him and how they are hurting themselves with their own arguments.

In the 1983 Watchtower 4/1/31 it says,

"This translation was used occasionally in support of renderings of Matthew 27:52, 53 and John 1:1, as given in the New World Translation and other authoritative Bible versions. But as indicated in a foreword to the 1980 edition of The New Testament by Johannes Greber, this translator relied on "God's Spirit World" to clarify for him how he should translate difficult passages. It is stated: "His wife, a medium of God's Spiritworld was often instrumental in conveying the correct answers from God's Messengers to Pastor Greber." The Watchtower has deemed it improper to make use of a translation that has such a close rapport with spiritism. (Deuteronomy 18:10-12)"

"Has the Catholic Church ever been in communication with spirit beings from the next world?...In the history of the Catholic Church there are many accounts of messages received from the souls of the departed. The truth of these accounts is subject to the ordinary laws of historical criticism, and some accounts have certainly been proved doubtful. Others leave no room for prudent doubt. As a rule, God permits a soul only occasionally to communicate momentarily a warning, or a request for prayers, but nothing fantastical. Likewise, the messages are spontaneous, and not due to the curious efforts of people seeking the truth from the dead. The Church tests the messages received, or claimed as received, in order to discern whether good or evil spirits are responsible for the communication. (1) The message must in no way conflict with Catholic teaching or moral principles. Gal. I.,9. . . ."-Pages 73, 74 of the book Radio Replies, by Rumble and Carty, with the Imprimatur of Archbishop J. G. Murray, of St. Paul, Minnesota, February 11, 1938, and with a Preface by Monsignor Fulton J. Sheen, D.D. (9th printing of 1939)

"The words of Jehovah are pure words, silver tried in the furnace of the earth, purified seven times. Thou, Jehovah, wilt preserve them from this generation forever." Psalm 12:6, 7 Darby

"The first two, B.F. Westcott and F.J.A. Hort discuss their club, the Ghostly Guild, in their biographies. They were actually the fathers of the New Age channeling movement; their Ghostly Guild evolved into the infamous Society for Psychical Research.
The third translator to confess involvement was J.B. Phillips, author of the Phillips Translation. His autobiography details his necromancy and communication with the dead.
The last and most recent revelation of "giving heed to seducing spirits," concerns Dr. Virginia Mollenkott of the NIV. Her book, Sensous Spirituality, tells tales of her spirit guide and contact with her dead mother...The doctrines of devils which follow her includes admitted involvement with divination through use of Tarot cards and the I Ching." 'Which Bible is God's Word' by Gail Riplinger, p.81

Origen's six column Old Testament, the Hexapla, parallels O.T. versions by Theodotian, Symmachus, and Aquilla, all three Gnostic occultists." Ibid, p.79

"But is it only the pride of man that prompted this rewriting, or is some malignant supernatural power guiding the hands of these people as they take away from and add to the Word of God?  Below are some of the results of this satanic assault on the Bible."

"There's nothing "new" about the NKJV logo. It is a "666" symbol of the pagan trinity which was used in the ancient Egyptian mysteries. It was also used by satanist Aleister Crowley around the turn of this century. The symbol can be seen on the New
King James Bible, on certain rock albums (like Led Zeppelin's), or you can see it on
the cover of such New Age books as The Aquarian Conspiracy. (See Riplinger's tract
on the NKJV.)"

"Someone may ask, Do you then claim infallibility and that every sentence appearing in "The Watch Tower" publications is stated with absolute correctness? Assuredly we make no such claim and have never made such a claim. What motive can our opponents have in so charging against us? Are they not seeking to set up a falsehood to give themselves excuse for making attacks and to endeavor to pervert the judgements of others?" Zion's Watchtower and Herald of Christ's Presence, Sept 15. 1909, 4473

"If it wasn't for the JWs quoting from his translation, Greber would have rightfully faded into obscurity."

Greber's NT uses the Cross, in translation and imagery, something that resembles the Bible of mainstream churches more so than the New World Translation (who never use the Cross). By attacking Greber's NT, opposers are actually causing damage to their own belief system. Remember, the WT has only used Greber for 2 scriptures. Those that assert that Greber's NT resembles the NWT 100% (and yes, they do say that) are dishonest and lying. A closer examination (like the Cross) will help you see that Greber has much in common with non-JW translations.
Consider also Matthew 1:23:

 "Look! The virgin will become pregnant and will give birth to a son, and they will call his name Im·man'u·el," which means, when translated, "With Us Is God."

Did Greber's use of "a god" influence how the translation of John 1:1c was rendered by the New World Bible Translation Committee?
A quick check of the 1950 first edition reveals that this is not so. In the appendix we have mention and use of Smith and Goodspeed's An American Translation, Moffatt's Bible, A Manual Grammar of the Greek New Testament by Dana and Mantey, The New Testament in an Improved Version upon the basis of Archbishop Newcome's New Translation: with a Corrected Text, 1808 and Acts 28:6 in the King James Version etc. Of the above, only Newcome's Corrected Version reads "a god" at John 1:1c, while the other examples were used to promote the qualitative aspect of the anarthrous QEOS.
[It should be noted too that only the last clause of John 1:1, the "c" part, really resembles Greber's NT. The first part of the 1950 NWT start's, "Originally the Word was..."]

Are there others that have used "a god" at John 1:1c?
Yes, take note:
In a beginning was the Word, and the Word was with the God, and a god was the Word.
    (Interlineary Word for Word English Translation-Emphatic Diaglott)
    Harwood, 1768, "and was himself a divine person"
    Thompson, 1829, "the Logos was a god
    Reijnier Rooleeuw, 1694, "and the Word was a god"
    Hermann Heinfetter, 1863, [A]s a god the Command was"
    Abner Kneeland, 1822, "The Word was a God"
    Robert Young, 1885, (Concise Commentary) "[A]nd a God (i.e. a Divine Being) was the
    Word"
    "In a beginning was the [Marshal] [Word] and the [Marshal] [Word] was with the God and
    the [Marshal] [Word] was a god." John 1:1 21st Century NT Literal
    Belsham N.T. 1809 “the Word was a god”
    1928: “and the Word was a divine being.” La Bible du Centenaire, L’Evangile selon Jean, by Maurice Goguel.
    Leicester Ambrose, 1879, "And the logos was a god"
    J.N. Jannaris, 1901, [A]nd was a god"
    George William Horner, 1911, [A]nd (a) God was the word"
    James L. Tomanec, 1958, [T]he Word was a God"
    Siegfried Schulz, Das Evangelium nach Johannes, 1975, "And a god (or, of a divine kind) was the Word"
    Madsen, 1994, "the Word was <EM>a divine Being"
    Becker, 1979, "a God/god was the Logos/logos"
    Stage, 1907, The Word/word was itself a divine Being/being.
    Holzmann, 1926, "a God/god was the Thought/thought"
    Rittenlmeyer, 1938, "selbst ein Gott war das Wort" [itself a God/god was the Word/word]
    Smit, 1960,  the word of the world was a divine being
    Schultz, 1987,  a God/god (or: God/god of Kind/kind) was the Word/word].
    John Crellius, Latin form of German, 1631, "The Word of Speech was a God"
    Greek Orthodox /Arabic translation, 1983, "the word was with Allah[God] and the word
    was a god"
    Robert Harvey, D.D., 1931 "and the Logos was divine (a divine being)"
    Jesuit John L. McKenzie, 1965, wrote in his Dictionary of the Bible: "Jn 1:1 should
    rigorously be translated . . . 'the word was a divine being.'
Others, like Vine's and Harris have recognized that the rendering "a god" is grammatically possible. So we can see that the New World Translation does indeed have support for its rendering and does not need to rely on Johannes Greber and his New Testament. We simply do not know where his ancient manuscript, the Codex Bezae stopped, and his "spirit world" began.
For a fuller expanded list of various translations of John 1:1 click here.

Is it unusual for spirits/demons to recognize the truth of God's Word?
No, and this is borne out by the scriptures themselves. Consider Acts 16:16:

"And it happened that as we were going to the place of prayer, a certain servant girl with a spirit, a demon of divination, met us. She used to furnish her masters with much gain by practicing the art of prediction. This [girl] kept following Paul and us and crying out with the words: "These men are slaves of the Most High God, who are publishing to YOU the way of salvation."  This she kept doing for many days. Finally Paul got tired of it and turned and said to the spirit: "I order you in the name of Jesus Christ to come out of her." And it came out that very hour." NWT

"there met him two possessed with demons... And behold, they cried out, saying, What have we to do with thee, thou Son of God? art thou come hither to torment us before the time?" ASV

"And in the synagogue there was a man, that had a spirit of an unclean demon; and he cried out with a loud voice,
Ah! what have we to do with thee, Jesus thou Nazarene? art thou come to destroy us? I know thee who thou art, the Holy One of God." ASV

"Where is the child who is born to be king of the Jews? We observe the rising of his star, and we have come to pay him homage." Matthew 2:2 New English Bible

"Thou believest that there is one God; thou doest well: the devils also believe, and tremble." James 2:19 KJV

So what is the greater evil I wonder. The indiscretions of certain men like Greber (and perhaps those others mentioned above), or those who would keep others away from the Bible? By condemning certain versions, are you not condemning God for his inability to maintain divine providence? The mindset of those who condemn the New World Translation too often mirror the anti-intellectualism that runs rampant in the KJV-Only crowd. These people are not only anti-intellectual, they are anti-Bible and anti-God. It is these kinds of tactics, this kind of fear-mongering that brings out the worst in people of faith. It is okay to criticize certain elements of translation...I do it all the time. But I would never condemn the use of the King James Version, the New International Version and certainly not any Catholic version. They are all the word of God, and we can thank God for protecting his Word the way he has.

The Best Bible is one that is read!