Its better than it used to be
Crossover Games has improved over the past half year. Still has a few stability issues. Steam (one of the most popular applications used on crossover) tends to glitch every once in awhile; however, for the most part runs really well. Emulates Half-Life 2, CS:S, Gary's Mod pretty well. Left for Dead is a little taxing for Crossover. If your looking for games in the realm of source, Crossover will work just fine. If your going to play newer games I would suggest Windows XP on bootcamp.
BTW: On Crossover I am running a 8600M GT with 4gigs RAM and a 2.4GHZ processor and play Half-Life 2 on full specs.
Bootcamp FPS / Graphics Intensive games 2008 and Newer. Example: Oblivion kills Crossover.