This is a metasearch engine? It gets some pretty crummy prices, and it seems to highlight some pretty pricey choices. And it isn't obvious how to stop a request for updates to the price after you make one.
For the frequent traveler, this add-on by Yapta is essential. Every time you're looking at flights it just shows up and allows you to tag/watch those specific flights, thus avoiding (a) a separate trip to the Yapta site to utilize their very unique service, (b) having to go back again and again to the flight listings and check it. They notify you.
You know how some add-ons are kind of complicated and you're not really sure how to use them? Nothing overdone on this add-on. It's simple, it works, and it saves you time. Plus it's not intrusive to your browsing experience.
I did find a couple small issues, but no show-stoppers and the folks at Yapta have been responsive. Probably already fixed.