Done it! Well, parts of it between Mont Saint-Michel and Le Mêle-sur-Sarthe. The areas the route itself goes through are beautiful, and the surface varies from quiet paved roads to compact pea gravel on dedicated greenways that were suitable for our 25mm tires in the dry. That said, we tended to prefer the smaller quiet roads that wind through the villages rather than the bike-only greenways for a few reasons. The greenways tend to pass through the fields and skirt around the villages, so unless you purposely leave the trail you see slightly less. They're also pretty flat. A bit too flat really, we welcomed the small hills as an opportunity to ride out of the saddle and mix things up a bit. Also not having to navigate around the wooden barriers at every greenway-road intersection was a definit benefit of taking the road.
I'd definitely recommend the route as the countryside is beautiful, but don't limit yourself to sticking to it, especially on the greenway sections. No worries about traffic on most of the road alternatives either. There were some sections we didn't get overtaken for an hour or more at a time.