Hi Dan
I bought the NAS on Amazon around 5 months ago (£145) and it's been very good so far. It is quiet, but not silent. Makes a slight whirring noise. It was a doddle to set up using the software provided. 20mins and it was working. Transferring files is also a doddle, (drag and drop or use Goodsync etc). The first time I transferred my files across it took a while as there were several gigs worth. I use Goodsync to back up to a folder on the PC(temporarily). We access it using 2 laptops and the Xbox with any issues.
If you have it on auto mode it will start up when you power up your PC but it doesn't when I power up the xbox360 which I use for streaming at the moment, I assume that would be the case with the Sonos too, just switch to on and it works fine. Streaming video and music to the xbox is fine.
One very small niggle I find is that when I use MediaMonkey to access the music it doesn't recognise the NAS until I click on the NAS in my computer in windows explorer, then it works fine.
Any deleted files go into the Trashbox on the NAS which is cool if you want to restore an item but the trashbox can get very large. I'm not sure if you can set it to empty periodically.
I'd recommend based on my experience.
Good luck