Not really. I mean, most of the games he reviews are things I'm not interested in buying anyway, and, when he does review a game I want, I've usually already bought it before his review is out.
I've actually bought a couple of games he's given unfavourable reviews to after the fact purely out of curiosity, either because I'm interested in the concept enough just to give it a try despite its flaws (eg Assassin's Creed), or because I'm intrigued to see where the complaints (from more people than Yahtzee) are coming from, and see whether I agree with the defenders or the 'haters', for lack of a better word. That's right; I go out and buy games that I know people think are bad for the sake of analysing why they're bad, or to see what works and what doesn't.
I don't think Yahtzee has ever actually influenced my purchasing decisions. I'm pretty sketchy with what I'm willing to spend on games anyway. If I'm up in the air about something, chances are I'm not going to buy it until long after the price has gone down anyway.