I'm very tempted to buy it, but after looking it over in the store I have some reservations. My biggest problems are that many of the classes are seriously unbalanced and that many of them aren't sufficiently different from wizards or sorcerers to warrant being a core class of their own rather than a prestige class.
OTOH, it may be worth buying for just the arcane monk, and the tinker. The arcane monk may be a bit too powerful, but the concept is cool. The only objections I have to it is that the 1st-3rd level spells mysteriously jump from 4/day to 6/day at 18th level, which looks like a typo to me, since otherwise the progression is the same as the Bard's. I also hate that their spells are based on strength! Ki is internal discipline and focus, not overt physical power, and all previous Ki powers in D&D have been based on Wisdom.
The tinker, OTOH, looks extremely well balanced. I havn't looked at the item creation rules, so they may not be, but otherwise they look like arcane clerics who need a spell book for a very limited spell list, and some nifty item creation and craft skills.