I am not getting the Firefox problems others are reporting (I am UK) and find this extension incredibly useful. Even if it does collect data on what you're interested in it will not be any more than all the other sites you go to anyway so if it saves me money (and it has) it's well worth it.
I really like that for 3rd parties it includes postage in the calculation (unlike other similar sites or plugins). My only reason for dropping a star is not being able to take UK Amazon wishlists automatically like it can US ones and it's a long process (unless I'm missing something) to go through and remove the tracking once you've bought the item.