We did it from Roscoff to the Dune du Pilat a couple of years ago en-route to the Med and Spain.
Roscoff to Nantes, and Nantes to Bayonne are completely different.
Roscoff-Nantes is inland, a few bits of road, cycletrack on disused railway line, but mainly canal side trail, quite nice but untypical of the Velodysse as a whole.
Nantes to Bayonne I would say is much nicer, particularly if you like the coast. If camping as we did there are loads of sites on this stretch, but as robing says, you need to avoid the French holidays, as all coastal areas in France are absolutely rammed then - and campsites normally put up prices - near the coast they can easily more than double.