The Tremsetter works OK, but it's not a cure-all. If it's set up correctly, it will pull the strings/vibrato assembly to what feels like a detent, right in the center of the vibrato's throw. So it's supposed to bring your strings back to the same spot every time.
I find that it works fine for mild to moderate vibrato work (but having no Tremsetter at all might be OK too). Deep divebombs will throw mine out of whack every time. I think the roller nut ain't rollin'.

If I were to want to do serious whammy work I'd probably have a luthier install a Floyd Rose with fine tuners, and a locking nut.