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.
If it's your honest opinion, then I would compliment the talk. However, I wouldn't do it just to be polite. There is no need for false kindness.