The book has a variety of authors, and has a very uneven level of quality. Further, from a little behind the scenes chatter, I have seen that some ideas that were fine when written up got butchered by the editors.
There are some pretty good ideas. For example, I never thought I would like the undead PC race, but they are pretty neat and I would put them in a campaign in a heartbeat.
Mechanically, many of the races and classes are fine as is or can get by with some minor tweaks. But some break from convention with no good reason, and some are mechanically very broken.
The most broken thing in the book is, perhaps, the
Uthuk Qi'nok -- a class that has both figther BAB and full spellcasting advancement.
Other things you may want to change before using is things like the
faeries. Instead of penalizing the character heavily in the realm of levels, faeries are limited in HD to a d4. The editor argues that this is a fair method of limiting them. I disagree. It works great for limiting fighter types, but then, considering their physical characteristics, there are few players who would play one as a fighter type anyways. But if you are a spellcaster, there is little to discourage you.
Still, there are many good ideas and some races are usable as is, and the presentation is very nice. If you want a sampling of what the book is like, I suggest you download the web enhancement/preview:
http://www.fantasyflightgames.com/pdf/mr_extra.pdf
Hope this helps.