For me, at least, the games he reviews that I haven't played yet are games I likely have no intention of playing in the first place. As such, episodes for games I haven't played are just for the laughs for me because he is indeed one hell of a good writer (my favorite line of his isn't even spoken, it's the part in the Alan Wake review when he's describing how you find little snip-its of story on the ground that tell you what is about to happen: "And then Alan was savaged by the biggest and most sexually frustrated bear ever.")
For games that I have played that he has reviewed, seeing as how he has a week minimum between the release of a new game and his review of said game, nothing he says can have an impact on whether or not I have already bought the game. That said, however, when he does review a game that I had bought a week before, I do often find myself either nodding in agreement, thinking "That's exactly what I felt about the combat system" or whatever, or thinking "You know, that's actually a very good point about the boss fights" or whatever in the case that he mentions something that I hadn't noticed.
And finally, for games that he reviews that I haven't played yet but do intend to get, I don't let his judgment sway my opinion unless he gives it just an absolute shit rating (such as Wet, which I haven't played). Yahtzee knows why people watch his show, they wanna hear him rip a game to shreds and shove a cactus up its ass. That's why even in games that he does like he always has grand hyperbolic rants about every negative thing that can be nit-picked from said game. So seeing as how he starts with the objective of tearing a game a new asshole, I don't let his reviews deter me from getting/playing a game that I was planning on getting/playing.