I do liek yahtzee, his videos are hilarious, and I do actually agree with a lot of his criticisms. But he does greatly exaggerate them generally (which is ok, otherwise the reviews would be fairly boring). For example, with Halo Reach, I did agree that some of the missions seemed kinda pointless, and that the spartans were uncharacterised, but unlike Yahtzee, that didn't wreck the game for me, it was still lots of fun.
So in short, no he doesn't affect my purchases, or at least not individually. I'll often look at several reviews before i buy a game, but if I've made up my mind then i'll usually buy it anyway.