I have a Qnap and never had any problems with it.
I host my website on it. It does surveillance for the house as well.
I have mc2xml running on it. It updates an xml file every day that I use for my TVguide on the Pronto.
It runs 24/7 so if you buy a Nas that is running 24/7 then go for the HDD that are made for it. I use the WD1002FBYS. They're 24/7 Nas drives. It's been running without any problems for over 1 1/2 years now.
A good NAS is almost like a computer without a keyboard and screen but running a stable linux version.