Did the Trans Cambrian three years ago in September and despite three days of headwind and rain (ok, the sun came out when waiting for the train at Dyfi Junction!) had a really, really good time and saw no one apart from a farmer when we pushed up at the edge of a ploughed field.
Route finding using the IMBA maps (a tenner if I remember correctly) was fine and there are plenty of options to cut it shorter if weather/mechanicals/general malaise hits.
We stayed in random accommodation recommended on the IMBA info sheet which was great and carried our stuff in largish Camelbaks (containing minimal overnight change of clothes, bad weather jacket, knee and arm warmers).
We are/were far from awesome atheletes but reasonably fit and I reckon 3 days is a good time to take – we rode from 10ish to 4ish each day with a variety of breaks, had a change and wash of cycling clothes at the B+B and then a pub meal and a couple of beers.
The terrain is not at all radical, more old skool out in the middle of nowhere riding (although you're never that far from somewhere in reality). Stunning though, especially the Elan Valley.