Hi Rich,
I actually started a thread on this sort of subject as I have a mate who wants to play file based music but he isn't too IT savvy (not suggesting you're not by the way), and the touch with an HD plugged in looked like a nice simple starting place.
The thread was 'help me save my mates from the evil world of brennan...' bit of a stupid title I'm afraid, must have been down the pub beforehand
But there is some info there, I know there are some limitations when you use the Touch as both server and player, I think there was a link to the 'slimdevices forum' on that very subject...playlists are limited to 100 tracks for example. So it all depends how you want to use it.
I'm running a QNAP TS-119 1.2Ghz Turbo NAS as my server (7.3.3 more efficient on a linux platform than 7.5 which the touch runs), plugged directly into my Sky router, and have 2 receivers, and one classic throughout the house. Which is very nice for me, but a bit to stomach initially.
For the price of the Sonos you could add an additional DAC to the Touch, and a really nice amp, and Sonos does not support Audiophile ReplayGain.
I think the Sonos is just a wireless enabled Sony music centre, but then again I may be biased.
If you wanted to do it cheaply, you could use an old second hand laptop as your server plugged into the router, it doesn't need to be very powerful.