For what its worth, I've had issues over clocking my U1. It will happily overclock to 2.0 GH/S but anything over that and it errors out quite quickly. But when its running at 2.0 is has a staggeringly low number of errors as compared to the old 336 mh/s Block Eruptors that I've used in the past.
That is where all my U1's and even U2's (apart from some @ 2.1 and some others @ 1.9) spend all their time stable. other's can do more with voltage modfying but i haven't a clue what that invloves. With the price those miners are, 2GH/s is a pretty good value and easy to setup.