Just starting using Avira and I like it. I switched from AVG because it game me a couple of false reports (Intially when installing Bioshock).. Went through a few free trials.
Norton is so, obtrusive, and it never uninstall cleanly. Kaprensky I found to be a poor performer, it seemed to to take up a bit of process power.
I'll stick with Avira for abit, but, might flip back to AVG because of the popularity of it.