I use to have two MSI R9650 Twin Frozr III's running in crossfire. Performance was excellent, however; screen tearing and micro-stutter eventually started to annoy me.
Also, games on release day are often not well optimized for multi-gpu setups. So i ended up having to wait a few days or weeks for patches to be released before the cards worked very well. Off the top of my head i had this issue with (Crysis 2, Skyrim, and Max Payne 3). When the Radeons work together, they worked very well and gave me great FPS. But as i mentioned above, micro-stutter became a nuisance over time.
A few months ago i sold the 6950's and purchased an MSI N670 Power Edition. It doesn't quite give me a the speed/fps that the crossfire setup did, but the single GPU solution is great because there is no micro-stutter and more often than not there are not driver issues the day new games a released.
Cheers
Edit:
As far as selling the GPU's, i sold one for $200 and one for $170 (4 or 5 months ago).
I also agree with the 'Never Settle' bundle comments. While i am very happy with my Nvidia N670, the AMD game bundle is crazy good.
And i highly recommend any MSI Twin Frozr gpu's. They are great coolers.