It is new, (opened this year) but I think all the waymarkers are in place now, and I have heard the Cicerone guide is very good.
There is a fair bit of tarmac, but if you took OS maps along, you could avoid a lot of it. You can pretty much go where you like as long as you aren't in someone's garden. I have heard good reports of the route.
The bits I know are partly new paths and partly traditional bog. In some areas you are just signposted across moorland with no trace of a path, and you will get wet feet. It's easy to wild camp, but if you want fixed accommodation you would need to book ahead, the islands get very busy in summer. The Gatcliffe trust hostels have a tradition of cramming you in somewhere though.
You won't meet many people along the route, but you have a very good chance of otters, eagles, hen harriers.
Spring comes quite late to the islands, be prepared for very mixed weather!