True, 90% of anti-virus is the user, not the program, but I like when that 10% does its job well. I used to believe Avast free edition was pretty good, but after some testing, I found that it wasn't so great. I usually create a partition, install an AV, then let loose some viruses to see what happens. I benchmark the resources used by the AV as well, which I believe this to be a pretty good test. If I remember correctly, Avast failed on even the most simple of things, like the EICAR test.
You can do this yourself by simply opening a new text document, add the following line...
X5O!P%@AP[4\PZX54(P^)7CC)7}$EICAR-STANDARD-ANTIVIRUS-TEST-FILE!$H+H*
...and pressing "Save". It is not actual virus of course, it just has similar heuristics, so no harm will come to your PC. There are plenty of actual "tests" out there you can do, but I personally prefer to do my own against actual malicious software, I think it is a little more accurate.