My folks lived in Lewis and Harris, and I've taken a Monster and 900ss there and to N.Uist; Monster slightly better on the roads.The roads are very variable and you'd really be going there for the scenery rather than the riding.
B&B tends to be along the lines of the mainland, polyester sheets etc, but certainly clean and cheapish in my experience, and hearty breakfasts. There were a couple of upmarket restaurants with accomodation in the wilds, Scarista House in S.W. Harris being one of them. The Rodel Hotel in SE Harris has apprently been done up since my last visit. Worth a visit in a gruesome kind of a way on a Friday or Saturday night, though I got my one-and-only breathalyser on the way back home on a Friday.
Wouldn't go quite as far as others on the horrors of SY; might be worth going there during the Celtic music festival (every July I think), late licences and a good atmosphere I thought.