I run Beryl on my main workspace. If it ever crashes it falls back to metacity (or kwin). As long as you stay modest with the plugins (stay away from rain effects, blur effetcs, etc.) it's stable enough to work with. Further, the Compiz and Beryl communities are getting back together, so we'll see the stability of Compiz and for those who want the extra plugins, the eye candy of Beryl (named Composite Community).

Beryl and the Beryl manager tend to take up anywhere between 20-50MB of RAM on my system. Blur is the only real plugin that kills cpu cycles. I always disable it.