I have recently bought an 2nd 7970 to support my other 7970.
Before i bought it i have seen people with "frame-time" issues with crossfire solutions. I wasn't to worried about it becouse i had no issues with it when i had 2x 5970 in xfire.
But after installing my 2nd 7970, I also noticed some serious frame time issues. 60 fps feels like 30-40ish sometimes. After upgrading to the latest beta driver it has improved some but a single 7970 feels much smoother at 60 fps. Enabling vsync and capping the fps also seems to help a bit.
I hope AMD addresses this issue in their upcomming drivers.