I'll be honest. While the USB devices "do capture video," at a capture resolution of 320X240 it looks pretty blurry when put back to tape. I have the XLR8 InterView, and have seen a few others in action. The probem stems from the capture resolution of 320X240. It doesn't allow you any extra pixels to sharpen, blur, or do any effects to make it look any better.
I believe capturing at a higer resolution would be ideal, so I'm holding out for the Sony convertor, or a DV camcorder.
It'll work [USB] for home projects, but not much more.