I didn't have any issues with the game. It actually run pretty good on my cards even in surround (when I had it).
It could've been that I was playing a pirated version that didn't have every patch.
Speaking of which, pirated Far Cry 3 is stuttering/lagging terribly on crossfire. I was showing 50fps on Ultra at 1440p but still too jumpy/laggy. I had to disable crossfire (and therefore bring my settings down a little bit) to get it smooth.
I'm sick of crossfire more than I am of AMD in general. It's a cost-effective way of upgrading your video card if you've got the power/board overhead to support it, but it seems like 75% of the games released in the past few years aren't optimized for it. They either don't use the second card at all, or it's jittery/laggy and annoying.
About ready to pull the trigger on a single GTX 680 and hopefully be set for a while.