I was lucky enough to pick up and A4000D with a Deneb card inside and I'm amazed at how well (and how fast) it is.
The difference between USB 1.1 (Subway) and the USB 2.0 of Deneb is massive. If you're planning on using your USB for mice and keyboards only, then you'll be fine, but using flash drives or external USB hard drives or CD drives and you'll suffer...
I have to say that the Deneb would have to be one of the best cards ever produced for the Amiga. It really opens up a whole new world of connectivity. Of course the excellent Posiedon USB stack is a big part of it too...
Cheers,
Mike.
P.S. I also have an A3000D which I might try the Deneb in, just for fun! :-)