Went to the Loire with my wife a couple of years ago, northern reaches from Blois to about Angers. Was nice, but felt a bit disappointed with the area. Plenty of beautiful Chateaus and many towns were picturesque, but found the cycling a bit uninspiring and the river and surrounds were a little industrial, but not in a nice way. Can't comment on further upstream, but not the romantic place i had imagined. that said, Amboise and Chinon are both great places and the wine is brilliant.. worth visiting one of the many wine caves. PLenty of marked cycle routes too, mostly flat with the odd steep climb up to and through the vineyards.
Cycled the Rhine north of Frankfurt this year and that was kind of what i expected of the Loire (castles, scenery, vineyards etc)