Is it desirable to compliment a seminar speaker before asking a question? Some people, when asking a question at the end of a seminar, make comments on the talk first along the lines of "great talk," "really enjoyed this talk," or similar phrases. Is this a desirable thing to do? I've noticed that most people don't do this, but some of my favorite scientists do.
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.