With how easy it is to buy games from Steams, the indie bundles and the constant sales... I find it very hard to NOT buy games. I have around 300 games on Steam, I admit I have not even played a quarter of that. Part of the thing is that I sometimes buy indie bundles for a game or two but I end up getting like 4 other games I really have no interest on. Also, my connection is slow so downloading all the games takes a long time!
That said, I do sometimes download a... "backup" copy. My computer isn't that great, so I like to try to see if the games even RUN on this computer before I take the plunge. If the game runs and is cool, I'll buy. Though sometimes I just buy the game and then pray that it works, I did it with both Dark Souls on PC, haha. They worked great, compared to the console versions, so it's all good.