I've had this book since the week it came out and I've never used anything from it. I offered my players a chance to vote on using the critical hit system in here, and they were overwhelmingly against it. Their main complaint was that if they were using it against monsters, then monsters would be using it against them, and it was too easy to die on dumb luck.
Perhaps it does make combat more realistic, but when a single swing of a sword involves 6 dice rolls and takes about 10 minutes to consult all the right tables and figure out all the modifiers and effects, I think you're doing something wrong. But, hey, maybe some people like that.
The monster tactics section is more useful, and if a DM uses it properly, he allows his monsters to become more than a ball of hit points that the characters have to reduce.