we use them in addition to adwords. We base our use on cost of sale. For us, if we can get a shopping engine running at 10% or below for cost of sale, we stick with it. Some of these product feed sites just simply have mass quantity of users, amazon product ads for example, the cost of sale is for us is only just under 7% and the amount of orders we gain from there is pretty impressive.
Which ones have you found to be the best? Google Shopping is the only one I'm getting any sort of results from. I'm not using Amazon Product Ads since they're a competitor and can't be trusted not to use your data against you even with Product Ads:
I use to use google, until they changed it up, at that point the cost of doing business there was over my cost per sale line. For me pricegrabber, nextag and amazon product ads. Ive heard the horror stories about amazon, but ive been selling there for years with no issues.