BreOl72: Since September 2010?
Exactly zero.
Likewise, except since November 2010 for me. :-)
I guess I've been lucky in that my computers have managed to run just about everything I've ever bought here.
There have been a few really old games that have posed a challenge, but since I paid very little for them, it's hardly worth the trouble to refund. And besides, I've kept all my old PCs going back to 1993, so if I can't get something running on this rig, I'm pretty sure I can get it running on an older one.
That said, I have paid for some new games here as well, one of which I recently spent over $100 for. And if I just couldn't get one of those to run, I'd consider a refund I suppose. But fortunately, every full-priced game I've ever bought here has run fine, so I've been lucky I guess.