I got a MiniStack with no drive for $79 and it works as well as expected. My only complaint is that it is slightly louder than the Mini, however it is still remarkably quiet compared to my old G4. I put a Seagate 500GB (8MB cache) drive in the Ministack and at some point will probably use the internal 2.5" 50GB drive to install XP on just for kicks. I get around 38MB/s from the firewire connection which is about as fast as one would expect. I was supprised that the internal drive in the Mini got around 30MB/s which is reasonably good for such a small drive.
One other thing... when I shutdown the Mini, the Ministack does not shut off automatically, which I was thinking it should. This doesn't really matter to me as I leave it on 24/7 anyway, but it is odd.