haggul - 11:15 am on Mar 26, 2010 (gmt 0)
They can and do work but you need to be careful. Think about it - it's just the same as adwords really - you are taking as much of a punt with that.
Difference is porbably you only PPC advertise those high converting products, and of course you have a bit more control over the ad copy etc.
I bet your comparison site experience invloved giving them a full product feed - a better approach would be to distill that down to the converting products, or the higher margin ones - which is essentially how you probably tweak your PPC campaigns - see what isn't working and drop those.
Having said that we have stopped all but one of our comparison site listings - more because of the principle that we disagree with their Christmas price hikes than anything else, but they have remained off - we can see a small hole in turnover, but not neccessarily in profit of course!