I don't think I would leave a show during intermission. I would hate that feeling of "what if it got better..." that I *know* would be stuck in my head. That said, I completely understand if a person chooses to do so. Its there money!
Though, LouieLovesTheater, all mef said was that he thought it was terrible and left. I don't think you have a right to criticize him for saying that. If he gave a detailed review of the first act and problems with the book, score, and performers that might be another story - you never know what will happen in the second act. But, just saying that you left during intermission is merely a fact not a review.
I want to write music. I want to sit down right now at my piano and write a song that people will listen to and remember and do the same thing every morning...for the rest of my life. - Jonathan Larson. Tick, Tick...BOOM!