Google's spam filtering is excellent, although I get a lot of mail that is intended for other people by my name (I imagine these folks going "Hmmm, is Jim on Hotmail or AOL or GMail?")

However, my number one issue is a growing unease with having the content of my "personal" email mined to power targeted advertising. I half expect if I were to mention something like "terrorism" in an email that I would start seeing adverts in GMail for fertilizer.