Its smaller than the PSP, 40 gigs of storage and you don't need a PC to interface, plugs directly to the PSP USB port ... works great.
You have to move the files HD <-> Mem Stick
It came with a 15 in 1 card reader so I usally don't even hook up the PSP, just pop the memory stick in the reader.
HD battery life about same as PSP
I got it here if anybody cares
They have a 60 gig drive but it doesn't work with the PSP and I don't think it comes with the reader ether.