Is the NIV a good translation?

The NIV is a very good translation. It is, however, a "dynamic equivalent" translation. That is, they translate "thought for thought" rather than "word for word." That makes it more "readable" than the NASB and other "word for word" translations. This means that in some cases its translations do not precisely correspond to the actual Hebrew/Greek/Aramaic words. For instance, "mountains of God" is translated "mighty mountains" (Ps. 36:6), since most scholars agree that "mountains of God" is simply a superlative form of "mountains."

The NIV does contain a note suggesting the meaning of "behemoth." "Behemoth" is a transliteration (a simple substitution of English letters for Hebrew letters) of the Hebrew word in the text. Scholars aren't sure how to translate it since they don't really know what it means. The NIV suggests possible meanings, but does not assert any of them. Personally, I think that is more helpful than simply transliterating.

Many people have attacked the NIV for various reasons. Most of the attacks I have seen were based on poor knowledge of the original languages, many were simply alarmist, and quite a few were motivated by faulty research into particular issues. Most of the particular "bad translations" I have seen people put forth were actually decent translations that ran against the objector's preferred interpretations (interpretations that came across more clearly in other translations). This is not to say that the NIV is perfect, or that I support every one of its translations (or that my opinion is the standard by which a translation should be judged!). Nevertheless, it is a very good translation.

Perhaps the most popular example of this comes from the supposed "feminist" slant of the NIV, a charge brought about by the NIV's use of "gender inclusive" language. First, the NIV does not use "gender neutral" language. Today's NIV does, but that is a completely different translation that just happens to have (for marketing reasons, no doubt) a very similar name. The NIV does it once in a while, but not that often. Second, many modern Bibles use gender neutral language, including the NRSV and the NKJV. There is a good reason for this: Hebrew and Greek commonly use masculine pronouns to refer to groups containing both males and females. When translators believe that the original meaning is gender inclusive, and when the translation philosophy of the version supports it, they go with gender inclusive/neutral language in the English. This is not a bad practice usually it makes the original meaning clearer. For instance, the Old Testament usually uses "sons of Israel" in reference to the entire nation of Israel. In English, some might think that "sons of Israel" refers only to the men of Israel but that would be a wrong understanding of the text.

Then too, some people object to using anything other than the Received Text, which was the sole text used to translate the KJV, but which was only one of several texts used to translate most modern Bibles. Many of the verses that were supposedly "deleted" come from the Received Text but are not found in many others. In other words, the NIV translation committee was persuaded that these verses had been wrongly added to the Bible by earlier copyists of the Received Text.

Every translation has its weaknesses, but I don't think the NIV is more prone to error than any of the other good ones available today. If you are interested, I did a Q&A on Bible translations a while back: Bible Versions.