I should also mention that I use the Linkstation in a RAID-1. You might get better speeds with a RAID-0 or jbod, but then of course you don't get the data protection.

I currently own a Buffalo Linkstation NAS, it gets about 20 MB (megabytes) of traffic/sec on reads max, which isn't all that great but at least it's into GigE speeds. The web interface is decent, and since it supports samba and ftp it should work fine with OSX.
I also have tried the HP Media Vault (their small linux NAS), but I found that when I put a second drive in it would over-heat and turn off. The user interface sucked too. I didn't get a chance to benchmark its speeds before I returned it.