Just completed the route from North to South. You are on cycleways / rail trails or dedicated cycle lanes almost all the time. The route is well signposted for the most part (towns can be  a bit sketchy). For some reason my Garmin often jumped to the nearest road which could be confusing when on a cycle way. The sections along the Orne, Mayenne and Loire go through beautiful countryside. The condition of the tracks is also good for the most part. Really this is best suited to gravel tyres though.
I would challenge the concept that there are no hills or that the hills are easy.  The section round La Roche d’Oétre is hilly. Over a 73k day from south of Caen to Flers there was 814m of climbing. Fine if you are young and carrying nothing but that which you can stuff in your jersey pockets. More of a challenge if you are no longer young and carrying full camping gear. It is all flat after Nantes though. 
One thing to bear in mind is the GPX pre planned route is designed to minimize road sections. At times this means sending you off round 3 sides of a square only to reappear just down the road from where you started. I would suggest you review the route ahead regularly. If you are used to dealing with traffic and tend to be up and away early then there are more direct and sometimes smoother roads to be had than the GPX route. One section I would definitely suggest you skip is on the Domfront to Mayenne section. The route sends you off to Torchamp and Saint Fraimbault at a place called Le Gué. Alternatively you can save yourself 10k and some hills and stay on the lovely shaded rail trail all the way down to Laubrière. That said the routing as a whole definitely takes you round some scenic spots even if it does include the odd section of cobbles and boardwalk that will shake your fillings loose. 
If ,like me, you are camping, then many sites have facilities for touring cyclists like a fridge, microwave, kettle and in some places some huts for accommodation so you can be up and away quicker in the morning. Look for Accueil Vélo.

All in all a very accessible route with plenty of options and dead easy to access from the UK if you can get yourself to Portsmouth and a lift back from La Rochelle.