So what if they find fingerprints all over it? How do they go from there to creating a 3-D version of your print? The fingerprint scanner reads the ridges & valleys of your print from beneath the skin, so you won't be able to easily fool it. And, I've read that Authentec's patents (now Apple's patents) also include the ability to detect living tissue, so if that's in there too it would make it really hard to do.
And speed? You can doubt it will be fast, but a slow reader isn't something that I think Apple would ship. After all, they have a 64-bit fast processor that has nothing to do at the time but verify the print.
As for it being useful, about all it will get you right now is to unlock your phone and make purchases. That to me is useful enough (except now that I won't have to enter my password for every purchase I will risk eventually forgetting it), in the future it may be considerably more useful.