I rode the Ridgeway a month ago and it was in great condition. Hardly any mud at all. Of course that might not be the case now. I did actually comment to my riding partner at the time how it wouldn't be so fun in the wet.
Except for the part just near overton hill, which is very rutted, the remainder of the track is OK as long as you don't want to really race along it.
And don't listen to those above about illegal sections etc - you just need to combine it with icknield way and you're fine. We ride from Pewsey to Tring without riding on footpath once.
Cycle-touring, Expeditions, Adventures, Major cycle routes NOT LeJoG (see other special board)