As I recently recalled in response to a French getting-around question, "when in Rome, we do as the Romans do." In French cities of all sizes, I'm all for taking public transportation. In the country, like the French, you're much better off with a car.

IME, SNCF will get you to Caen easily but provides limited service to other Lower Norman cities. This might work to get between cities for stays, but won't like work as transit options for day trips. Certainly there are no cities other than Caen that are large enough to have an urban bus network, and I'm not familiar with anything other than ex-Paris tourist coaches that get to the beaches or American cemetery.

As for the Mont St. Michel, the Veolia buses run between there and Rennes, with no SNCF connection between the area you'd like to see and Rennes. You'd have to drive.

As for car rentals, there are a number of agencies outside the Caen train station. If the rates are terrible (as they can be in high season), check out the local Super U supermarkets, which offer cars at €5/day and €.12/km (magasinsu.fr).

I'm basing this off of my experience in Normandy, and its similarity to Brittany - I wouldn't wish anyone to use the public transportation here to try to see the beautiul countryside.

Peace.