I only have experience with Google's shopping engine. Feeds sent there can bring great results IF: you have a decent ranking (it IS google, after all, so rank is king) AND you have either the lowest prices or darned near it, AND you have a product that's the same no matter where it's bought (basically any mass-produced item).
Of course, different niches will produce different results. It could be a real goldmine or a real dud, depending on what item(s) you have in your feed and where they end up in the listings. Needless to say, a listing right in the regular SERPs is the king, but as always, there has to be someone who wants to buy the item in order for it to be lucrative.
Since Google Base is free to submit a feed to, there's no financial investment to lose if your rank sucks or you get a zillion clicks from window-shoppers. I'd be leery of any PPC comparison engine since they're set up to encourage clicks at all costs, which can bring in too many window shoppers and lower the conversion ratio.
Feed-making programs are around, but the only one I'm familiar with is made to work with Zen-Cart. It's a free plugin and can be set up with cron automation so you don't have to even think about it to have it update your feed automatically. I think it's called Google Base Feeder or something like that (I haven't bothered looking at that screen in a while...I used the set-and-forget option).
Google does have some absolutely stupid and insane rules like bans on proper Capitalistic sales phrases like "free shipping." They also don't allow affiliate feeds, and seem pretty good at enforcing that (judging by the lack of aff links in Base). Still, it's free, so might as well make use of it.
