I don't see as a good fit for primary NAS. The form factor doesn't support internal drives and the USB interface doesn't provide things like spindown control or SMART interface. This thing really shines more if you've already got a NAS on your LAN. What I see as good applications:
* Torrentbox (works best if you've already got a NAS but external drive via USB would work fine too)
* Web Server/Mail Server (just add 4GB USB thumb drive)
* Firewall/Router -- run some nice open source stuff on this and consign your existing consumer-grade router to just be a Ethernet switch
I use an old P3 running off a CF card for this purpose, both routers are just switches and wireless APs. This would be a nice power saver in a role like that. Add on print server abilities too.