Years ago, I used the Key Word Study Bible in an NASB translation and found it very helpful. Several months ago, when our Bible study group began a series in Proverbs, I picked up the NIV Key Word Study Bible, thinking that it would be useful to be able to get more information on some of the terms in Proverbs. Though I didn't use the lexical aids as much as I had thought I would for the study, it was occasionally very helpful to learn more about terms--especially when, on reading a passage together in the group, we found that a particular word was translated in very different ways.
The Key Word Study Bible is a handy all-in-one source for background on the words, especially for when you don't have a full concordance or dictionary nearby. But it would be difficult to use this as my default, go-to copy of the Bible, because there are so many numbers mixed in with the text that it's often hard to find the verse numbers. It's a very cluttered Bible, and not so much a general reader's Bible.