It does what claims to, but be prepared for MAJOR lag as it compares prices and whatever. I'm not kidding, it will render your shopping websites NEARLY unusable. I've taken to disabling the add-on until I need it to do it's thing. On Chrome, you can right-click the extension in the toolbar to disable/enable it, but Firefox requires you to go to the Add-ons manager. That adds a step that I'd rather avoid.
So, the performance/responsiveness hit costs it two stars. Otherwise it is useful for checking prices and finding/using promo codes. YMMV