I use Symantec Endpoint Protection (SEP), mainly because it is a requirement for getting on my corporate network. It does a decent job and is fairly unintrusive, yet since it is a corporate version it does not give me the control that I like (some settings are locked down and I can't change them). For the few computers I have that can't run SEP I use AVG, though I have noticed performance issues with it as it actively scans files.
I used to use the ZoneAlarm internet suite, but had to give it up when I went to 64-bit. I don't know if they have a 64-bit version yet, and they seem to have dropped off of the radar in most security suite evaluations (a few years ago they were on top!).
I like my AVG free.... I supplement with SuperAntiSpyware free.
I notice a performance dip during scans if I'm doing anything big. If I just surf the web or do my homework, I don't notice that it's doing anything.... I do have a pretty new machine though. If it were older I may notice more. It's gotten better since the big update in December though.
When you do things right, people won't be sure you've done anything at all.
Well on the funny side I would say a computer disconnected from the internet. But truly I like clamwin. I have tested it out by trying to go to some very shady sites and my computer was virus free. I have tried AVG and was infected within a weeks time. Kapersky is also a pretty good one however it is not free.