I was using AVG (paid, but... I put free on everyone's computer) until 8, and then they were sucking so much out of my system I had to get rid of it. Then I tried Vipre (eventually also registered) and that worked great for a year or so but started locking up my Vista machine so I got rid of it before the license even expired. When I bought wy new machine with W7-32 on it I went back to AVG with 9 and that seems to be working pretty good.
I like to use Panda Free ActiveScan as a hail mary when I get into tight spots, and have been thinking about whether I want to try their regular products. The nice thing about activescan is that it runs in the browser and is not usually affected by the viruses that kill the AV processes on your computer. The regular product doesn't run that way, but this one is good for emergencies and I have had it clean up a lot of junk that other stuff doesn't find.
I wouldn't touch a Symantec product even if it was free, although I have used their cleaning tools before with specific infestations.