It works. Google Shopping works better though. When I do follow a link from Google Shopping, invisible hand actually pops up and tells me other prices, even though it doesn't ever mention the site I'm actually on. This is puzzling; if it knows enough to show me different prices, shouldn't it know that this is the best price, and tell me that from the beginning of my whole search?
However, this is only in some cases. It's still semi-useful.