It's all about the drivers. I had no issues with quad sli and that was some time ago. I'd like to think they are further along with it than back then. I will admit that with the 7950gx2's the performance/price in quad wasn't there. It did work fine though for me.
Plus, I just don't think I could ever think of shelling out $900 for just some GPU's no matter how great. Just so much cash.