I sometimes watch LPs if I'm not sure whether or not I want to play a certain game. If I watch for a little while, and don't like what I see, I generally don't bother watching the rest of the LP, and I certainly don't purchase the game afterward. You can't always judge a game by its first level or so--just look at TSL, which had a slow start, but gets better as it goes--but that's how I roll.
Just my two cents.