Although NAS enclosures are a bit pricey, they are very low power..
I went from mini-ITX server to full blown 10 Bay Full Server, and now am managing with a basic QNAP TS209-II..
The thing that sold it for me was the 24/7 cost and ease of use..
On idle, the NAS uses under 10W, and only adds the usual HDD power consumption when they spin up.. My mini-ITX used 62W on average, my full server 125W on average, my NAS so far has hit 12W on average over a 24 hour period.
Obviously it helps if you can use it as a meaningful 24/7 server, so I have the following running on the NAS (other then just basic shares)
- email collection/storage with IMAP server and web front end
- Newsgroup downloader (NZBGet or SABNZBD+)
- Bit-torrent Downloader
- FTP Server
- Subversion Server
- uPNP Media server
Performance wise, I get 25MB/s read speed and just under 15MB/s write which is acceptable and well into utilising the gigabit connection, the newer TS219 is nearly double that, so not too shabby..
Cheap NAS enclosures are OK for approx 100Mbit rates and OK for basic file sharing/storage, they are just quick enough to stream HD media etc, so can be made to work well enough...