They know their Bibles. I don't think anyone can dispute this.
They know their Watchtower magazines very well. They know the Bible proof-texts given in the Watchtower very well, and the specific assigned passages for daily reading. But they do not have as much comprehension of the whole Bible, taking all its books and viewing them in context. My impression is that they treat their Watchtowers as equal to the scriptures, sometimes as even more authoritative.
How they interpret different parts of the Bible is another matter.
How they made their own translation also is another matter. For the most part, it is a fairly literal word-for-word transliteration. But it loses the power of the original poetic language, which better translators try to preserve where possible (in Psalms, the Song of Songs, etc.). The way the ideas are expressed, the style, the majesty, the rhetorical impact, the word choices, the rhythms: all covey the teachings just as much as the plain words. The Bible is not a prosaic textbook!
The peculiarities unique to the NWT are unjustifiable: the Hebrew version of the holy name would never appear in New Testament works written in Greek, and reverent Jews (such as Jesus and the Evangelists) would not use that name so freely as it appears in the NWT.