I played Half Life 2 via Crossover and Boot Camp.
Of course, Boot Camp gives you full speed... but as Half Life 2 isn't the most demanding game nowadays, running it through Crossover gaming does very very well.
I did the video stress test of Counter Strike, and IIRC the FPS were about 75% of what you'll get using the same hardware via Boot Camp, so not bad at all! Easily playable running at full res, highest settings. (this was done on a Mac Pro '08, GeForce 8800 GT)