I had a relatively cheap NAS – the Icy Box IB 4220. It was OK and a solid little files and media server but had a few limits for what I was trying to do with it. Namely speed of transfer (8MB/s, limited by CPU in NAS) and inability to backup direct from the NAS.
After working round them for a while and getting frustrated, I splashed out on what is massively overspecced for me, based on the adage “Better to spend too much and waste a little than spend too little and waste the lot”!
Qnap TS239 Pro 2
http://www.qnap.com/pro_detail_feature.asp?p_id=147
Very fast transfer speed over a cabled network, upgradeable RAID and lots of backup options (currently set up to automatically sync selected folders as soon as you plug in a USB drive to the rear port) and all sorts of functions – media server, FTP, www with PHP and MySQL, DNLA.
I’m very happy with it, rock solid, fast, full of features and upgradeable – but not that cheap.