I never compliment someone that I do not know well or just do not know unless I am really, really impressed. My compliments are a bit more free when it comes to family and friends.
My mom hates this about me. She and my sister have very trendy taste in clothing, and I don't, so my Mom will always ask if I like whatever she is wearing. 8/10 times I won't like it, and I'll tell her, but with an added -you should no better, that's not my thing- at the end.