Well since avast doesn't use heuristics in its on-demand scanner, something which many reviews only test (on-demand scans), then it is hardly surprising if someone mentions poor heuristics as it isn't used.
I think you need to take a look at the latest av-comparatives.org on-line results for On-demand comparative - August 2006. If without heuristics and overall percentage of 94.58% you think that is very poor, then you will continually be disappointed by many AVs. If lets say avast were to add heuristics (which it says it hasn't got any plans to currently) it would be only targeting 5.42% to attain 100%.
As has been said many, many, time detection rate is only one reason (an important one) to choose a particular AV. There are many who have tied AntiVir and come back to avast with its less than perfect detection rate. However the proof has to be in the eating, for me, no infections in over two years using avast.