Is it desirable to compliment a seminar speaker before asking a question?
My personal preference is not to offer a compliment in this setting.  It often strikes me as less sincere (off-handed or perfunctory).  If I really want to compliment someone, I tend to catch up with them later and start with something like "I really enjoyed your talk.  If you have a minute to talk, I have a question for you about..."  On the other hand, I know that some people feel like asking questions (especially if they are critical of some aspect of the work) can feel hostile, and so they find the compliment as a good way to off-set that.