I needed a NAS urgently. There were several NAS in the game, the Zyxel 220 for the price, the Qnap 209 for the email notifications, the Infrant ReadyNAS because someone told me it was excellent and the Qnap 409, another 4-drive NAS. After some considerations, I chose the Qnap 409.
Why not the 209 or the Zyxel? As 2-drive NAS systems they both had no path for online migration to higher capacity drives. Also adding capacity is much easier for a 4-drive NAS than for a 2-drive NAS. Having two 750gb drives in the NAS and adding a third one will double the capacity from 750gb to 1.5tb. Whereas with a 2-drive NAS you have to replace the drives completely. The Infrant had some discussions on their forums with failed drives which stopped me from buying the ReadyNAS (Also they have been bought by Netgear). So the Qnap 409 is on it's way with two WD 750 GP drives for an initial RAID-1 setup.
Hope they work with time machine, or I have to buy an additional time capsule 😉
Update: Time machine works.