I didn't attempt sli until recently with my gtx 460's. I have had a couple of other sli capable cards( gt 120, 8800 gts, gtx 275) but never attempted it. It was not as easy as I had heard to set it up. Had a problem with my pc crashing everytime i enabled sli and upon restart the sli option was gone. Turns out it was the realtek onboard lan driver that was conflicting. After picking up a cheap dlink usb ethernet adapter, I was good to go. Going to attempt crossfire with an hd 6990 and 6970 as soon as the 6970 arrives...wish me luck!
**edit**
Got it up and running. Hitting 100FPS on ultra with AA on 1920x1080 in open combat with lot's of action and 140FPS inside such as the subway on the Operation Metro level in battlefield 3. This is at stock 6990 speed of 830MHz(gpu) and 1250MHz(ram). 3dmark11 score P12858 at overclock of 930MHz(cpu) and 1375MHz(ram).
I am in love.