IIRC it’s nearly all on-road nowadays, though I think it runs on a well surfaced path alongside the canal most of the way between Newtown and Welshpool.
The section from Cwmystwyth to Rhayader is bleak and empty, and you’re unlikely to get a mobile signal. Between Rhayader and Newtown it’s mostly narrow and winding lanes with the occasional tight bend and short, steep pitch. Steep climb after Caersws and another stupidly steep climb from Buttington to the top of Long Mountain. After crossing the Breiddens at Bulthy (a much easier drag) and the descent to Crew Green it’s flat all the way to Shrewsbury.
Llangurig has a village shop (http://www.llangurigpostoffice.cymru/) and a pub but not much else.
Llanidloes has a Co-Op, Spar, cafes (inc. vegetarian at Great Oak Cafe, decent pub grub at The Angel) and lots of pubs. They like their beer in Llanidloes.
Newtown and Welshpool are considerably larger and have large Tesco and Morrisons stores along with the usual others.
I know of a nice campsite on the route, run by an old schoolfriend of mine, 1 mile East of Llanidloes – http://dolllyscaravancampsite.co.uk/
Would recommend the chip shop in Caersws.
Might be worth noting some bike shops – Summit cycles in Aber; Clive Powell in Rhayader; Brooks Cycles in Newtown and Welshpool; Dave Mellor, Urban Bikes and Stan’s Cycles in Shrewsbury.