It depends, how much data you gottahave to backup.
Most NAS do have a rather low I/O performance.
The Linksys NSLU2 f.i. has a peak I/O of about 3-4 MB/sec.
On the other hand if you “hack” it (which isn’t all that hard, really) you can get a 24/7 music server and more for the SoundBridge, and whatnot.
If you need better i/o performance, the linkstations or terastations do really look great. They offer internal and external ports for harddisks, plus a gigabit network plug on some devices, at least.