Heuristics is a good idea but I think Avast needs to improve its detection rate and scanning speeds.
I am currently testing AntiVir Free Edition with heuristics on high, and scan files on read and write. It seems very fast, faster than Avast and memory usage is very low.
Performing a full HD scan including archives is much faster than Avast without scanning archives.
On another note, I have been paying close attention to Jotti over the past 5 days and I have seen AntiVir detect way more than avast. In excess of 30 times AntiVir + others have detected things avast has missed . I doubt is was due to heuristics and it wasnt the same thing 30 times
I did see Avast sometimes detect one or two things AntiVir didnt get.
Im am not sure if the free Antivir is as efficient as the paid version, something I will ask later over at their forums.