It's definitely my favorite 3rd Edition sourcebook, by any company. If only for the concept it represents: official recognition of house rules. So many players and DMs are afraid to branch out and try new things, this is like a whole book endorsing that concept. Not to mention the fact that, as a game designer, seeing it all as Open Content rocks my socks off.
So far, we've used VP/WP, Armor as DR, Class Defense, Reputation, Action Points, Weapon Groups, Gestalts, and several other options. Currently in my Shackled City campaign we're using Action Points and Weapon groups as the only modifications, though I'm wishing I'd decided to use Reputation for this game as well (since the players' reputation in Cauldron is very much a factor in some of the encounters).
I can't reccommend the book enough. I mean, you almost never use everything in any sourcebook (the three core books not included). I've used a larger number of things from this book than I have almost any other.