I'm using a TS-119+ with a 2TB disk inside. It's sitting in my wiring closet.
It's running MythTV for my TV/UPnP server, MythWeb for programming, Mediatomb to serve photos/videos via UPnP, and mt-daapd as an iTunes Music Server. I use PS/3s for the TV front-ends, and Roku Soundbridge 500s, 1000s, and 2000s for the music players.
It's about as close to silent as you get - I think it's fanless (you can see I'm not concerned enough about noise to find out). And it uses about 6w when it's idling.
I got into NAS solutions after I figured out running my MythTV system 24x7 was like leaving a 100 watt lightbulb on all the time, even when I didn't need it. I measured my old beige-box PC with a watt meter: a continuous 95 watts. And loud fans.
The QNAP delights me. All I could ask for that it doesn't do well is transcoding. There's just not enough CPU for it. But that'll come in time with some other NAS unit, or with offloading it via scripting to a full PC or Mac, when I get around to it.