We've just ridden a small part - Rohan to Josselin and it was basically a tarmac surface and really easy to ride. Further west, near Chateaulin it looked more of a broken surface, and also there, where it follows a river, it really meanders so the road is much more direct - if that matters. We met two couples, both at least partly Australian! who both had the same book on the Canal - sadly I can't remember what it was called.
I did a couple of short sections just NW of Redon. The only advice I'd have is I'd planned to do it on my Audax bike but was glad I chose to do it on the MTB. Also, if you're used to road riding, is to keep your bum off the saddle as after 40 miles my ass hurt!