Crossfire is not for beginners and (in my experience) you will have different issues with every game you play using it. Be it crashing, performance, visual errors, frame dips, etc; Crossfire can be a real headache.
Back when I first X'fired my 6870's I was playing BFBC2 religiously. In one driver release I was getting 100fps smoothly, beautifully, but would BSOD occasionally. The next, 40fps jittery, but never crashed. The next, 100fps, jittery, visual errors. Some games, I've never had issues. Some games, I've never even gotten Crossfire to work at all.
I don't know how SLI's compatibility compares to Crossfire, but I'm not too happy with my experience with ATI over the past few years.
I am jaded. I am cynical.