They're great, aren't they? They let us all find better deals.
They have certainly changed the way most people shop for financial products, with more people than ever switching to lower-charging deals.
So what else do we need to know about them?
The Office of Fair Trading is concerned that some sites do not "deliver an impartial, honest service with accurate and comprehensive information".
It hasn't named any names but it has written to 100 sites after its review found many need to improve privacy policies and complaints processes. It also said some sites need to provide greater clarity about the way search results are presented, and clear identification of the business actually running the website.
Does that mean they're misleading us?
Yes. Some sites put price quotes at the top of their list from companies that pay more. Meanwhile other sites are owned by firms whose products they can recommend, questioning their impartiality.
So should we not trust the results we get?
If you're shopping around for a deal, use two or three sites to get a fair comparison of what's available. The OFT also advises people to "opt out" so that sites don't sell their information on to others.