I think think sometimes people are cruel and rude who use sarcasm. If you are talking to a stupid person and being sarcastic, they are not going to catch on anyway and you are not making them any smarter in the process. Just stop. With friends, hell yes, funny.
In my opinion, I think people who are sarcastic are dicks. I just roll my eyes at sarcastic people. Really that's all you got? Stating the obvious, obviously. Wow. Gee.