It was, uh, neat

Is it okay to diss a play or concert or whatever if someone gets you tickets for it? Maybe only if I was Larry David. I think it would make a good episode of Curb Your Enthusiasm or Seinfeld. Someone takes you to a movie or play, and then afterwards they ask you what you thought. Are you allowed to say you didn't like it?
On the one hand, any opinionated semi-snob feels it's beneath themselves to praise mediocrity. After all, there's the sanctity of one's taste and intellectual integrity to preserve. On the other hand, it does seem somewhat rude. I mean, here you are, enjoying the pleasure of a friend. Why go cause all that unpleasantness? Plus, they might not invite you out next time, and who knows, maybe they'll pick a winner (that last line sounds so much like something George would think).
I admit, I try to avoid the situation by asking the gift horse what they thought first. If they didn't enjoy it, I'm more likely to let loose with my true feelings. If they enjoyed it and ask me what I thought I'll try and find a few things I liked. Seriously, it's like someone asks you if you enjoyed the music at their wedding. What are you going to say?
I'm going to write this into an episode of Curb Your Enthusiasm. I know they supposedly improvise all that, but I'll send it in anyway.