I don't think I've ever bought a game on the cover alone, but the cover definitely has influence.
Generally, I do my research before buying a game -read reviews, impressions etc. Video games, espeically in NZ, is far too expensive a hobby to be burned on games that aren't good. So I normally know whther I'm going to buy a game, and the cover has little to do with a purchase - I will buy it regardless of the quality of the cover.
HOWEVER, if I knew I was going to buy a game, and there were multiple covers available, then I'd get the one with the nicer cover. And sometimes, a nice cover will make me pick up the box to read the back, so nicely designed boxes can make it more attractive and get a look in when it otherwsie wouldn't.