Quad SLI isn't really that impressive. It is beaten by 7900GTX SLI and X1900XTX Crossfire. Also, in many benchmarks it is beaten by single cards. I think Quad SLI is more of a marketing gimmick than anything.
My take on it is that its a great technology that hasn't been fully developed. I think that we are going to find as time goes on that having a SLi system for gaming is going to become the norm due to increasing graphics processing power from newer games. When this happens, I believe that proper resurch and devlopment will be put into QSLi and we will see a platform worth the price tag.
Unfortunately we wouln't see that until 2008.
Cheers
Cameron