mattb - 12:02 am on Aug 23, 2007 (gmt 0)
You can experience positive ROI from shopping comparison engines.
Make sure you have a good analytics package in place to track results. Additionally there are hosted services that will allow you to upload one feed for all the engines. They will then allow you to turn off products that have low return or high bounce rates. We have found that the bounce rate is somewhere around 50%. It makes sense to pull out those products with extramely high bounce rates. Like ppc, it takes additional work after the ad is turned on.