It's a challenge, getting it working properly, just like Nvidia's SLI.
I have a Sapphire ATI 4850 x2 which is easy to set up for CrossFire as it is 2 4850's on one card, but I actually brought it as it had 4 DVI outputs rather than the standard two. It makes it so much easier for me as I am running 3 monitors but, if I want an extra little kick, (only for testing purposes, not games, you understand) then I click a box, wait a few seconds for the system to sort itself out and I have one monitor using all of the available resources. I have to be totally honest though, I haven't really seen many benefits but it does give pretty impressive figures in bench testing, especially if I add the extra card I got, that I didn't need, 3 x 4850's, a lot of heat, some impressive figures but never really used.