Now that I have enough brews under my belt to feel like I'm pretty comfortable with certain styles of beer, I often have beers that are completely not my taste or seem like something's off. Usually I can place my finger on it (i.e., too much crystal malt in pale ales). Sometimes I can't quite nail it down (i.e., something's going on with the yeast/fermentation).
Even if I don't like a beer, I think it's absurd to bust a guy's balls about it. Obviously he knows how to make beer that people enjoy or else he won't be in business long. I usually take it as an opportunity to talk shop with the brewer. If there's something I don't like in a beer, I generally ask as much detail about it without letting on that I'm not a fan. This way I'll learn exactly what not to do in my own brews.
Either way, no need to be a prick about things regardless of what side of the bar you're on.