Welcome!
You ask a great question. It's possible to make your own mind up by looking at the WTS's Kingdom Interlinear. Just compare John 3:16 and John 17:3 as they read in the Greek with how the NWT translates them into English.
Also, anytime you see [ ] in the NWT, this is where they're inserted a word into the original language. Many times this is done just to make the wording clearer in English. However, oftentimes it's done to change the original text to better suit the Watch Tower's doctrinal stance, particularly towards Jesus himself.
There's an entry on Wikipedia for the NWT;
http://en.wikipedia.org/wiki/New_World_Translation_of_the_Holy_Scriptures#Critical_review
Here's a sample;
New Testament
Theologian and televangelist John Ankerberg accused the translators of rendering the NWT to conform "to their own preconceived and unbiblical theology." [ 39 ] To support a view of theology overriding appropriate translation, Drs. John Ankerberg and John Weldon cite several examples, such as the NWT's use of "for all time" in Hebrews 9:27: “And as it is reserved for men to die once for all time, but after this a judgment.” Ankerberg and Weldon cite Dr. Julius Mantey on this text as saying, “Heb. 9:27, which without any grounds for it in the Greek, is mistranslated in the J. W. Translation… the phrase “for all time” was inserted in the former versions without any basis in the original for it.” [ 40 ] Strong's Concordance defines the involved Greek term ?παξ ("apax" or "hapax") as either "once" or "once for all". [ 41 ] [ 42 ]
Dr. William Barclay concluded that "the deliberate distortion of truth by this sect is seen in the New Testament translation…. It is abundantly clear that a sect which can translate the New Testament like that is intellectually dishonest." [ 43 ]
Edgar J. Goodspeed, translator of the Greek “New Testament” in An American Translation, wrote in a letter to the Watchtower Society: “I am interested in the mission work of your people, and its world wide scope, and much pleased with the free, frank and vigorous translation. It exhibits a vast array of sound serious learning, as I can testify.” [ 44 ]
Dr. Bruce Metzger stated for the NWT of the Greek Scriptures that "on the whole, one gains a tolerably good impression of the scholarly equipment of the translators." [ 45 ] However, Metzger also cites NWT renderings as instances of translating to support doctrine, stating, "the Jehovah's Witnesses have incorporated in their translation of the New Testament several quite erroneous renderings of the Greek." [ 46 ] He cites the NWT’s comma placement at Luke 23:43 as “In the interest of supporting the doctrine of "soul sleep" held by Jehovah’s Witnesses.” [ 47 ] Another example Metzger offers is the insertion of the word “other” four times in Colossians chapter 1 “thus making Paul say that Jesus Christ is one among ‘other’ created things.” Of this insertion, Metzger states it is “In the interest of providing support of [Jehovah’s Witnesses’] Unitarianism” and that the insertion is “totally without warrant from the Greek”. [ 48 ] Dr. Bruce Metzger characterizes the NWT’s use of “Jehovah” in the New Testament as an “introduction.” He writes, “The introduction of the word ‘Jehovah’ into the New Testament text, in spite of much ingenuity in an argument filled with a considerable amount of irrelevant material (pp. 10–25), is a plain piece of special pleading.” [ 49 ]
Charles Francis Potter has stated about the NWT: "Apart from a few semantic peculiarities like translating the Greek word stauros, as "stake" instead of "cross," and the often startling use of the colloquial and the vernacular, the anonymous translators have certainly rendered the best manuscript texts, both Greek and Hebrew, with scholarly ability and acumen." [ 50 ]
Reachout Trust writer Tony Piper concludes it is not a "faithful translation of the Scriptures…", giving as examples Acts 2:42, 46 and 20:7, 11 and he objects that “the NWT translates it to read that the church simply shared meals together” rather than using the phrase “breaking of bread [...] to disguise the fact that the early church celebrated the Lord's Supper more than once a year.” [ 51 ]
Alexander Thomson wrote: “The translation is evidently the work of skilled and clever scholars, who have sought to bring out as much of the true sense of the Greek text as the English language is capable of expressing. ... We heartily recommend the New World Translation of the Christian Greek Scriptures, published in 1950 by the Watch Tower Bible and Tract Society.” [ 52 ]
Thomas Winter considers the Kingdom Interlinear Translation of the Greek Scriptures (part of the NWT project) as a “highly useful aid toward the mastery of koine (and classical) Greek”. Winter relates that the translation "is thoroughly up-to-date and consistently accurate.” [ 53 ]