I hear good things about Hauppauge...but I don't hear many good things about USB tuners, as compared to PCI tuners. Amazon reviews on the Hauppauge and Diamond PVR660 (two of the better ones) are lackluster at best.
But overclockers.com seemed to like Compro's USB tuner. Might take a look at that one.
Personally, I have a Compro VideoMate Gold+ card, and with the newer TV software, it's quite excellent. Better than anything ATI ever released, and on par with the picture quality from my old nVidia Personal Cinema card. So I suspect Compro's USB sticks may also be better than average.