I've recently added a feed to Google Base and am keen to do this accross some other comparison shopping engines - for example Amazon, Pricegrabber, Shopping.com, Yahoo etc.
However, whereas Google Base is free, the others require more time spent in setup and charge a fee for the service / or per click.
What experiences have UK users had with any of these feeds? Which would be the one that, from your experience, would be the one to start with?