"Mont St-Michel is a great place to stay overnight, as the daytime crowds disappear and the place is beautiful to walk around after dark.
The Auberge St-Pierre is a very nice little inn, but keep in mind this also means the rooms are fairly small, too. The toilette is in such a little room that a normal sized person can barely fit. Very nice shower, though, and the rest of the washroom is fine.
The "double" bed was two singles pushed together, each one made up with it's own bedding. Still, it was comfortable and the room was warm despite the single-digit (Celsius) temperatures outside.
The continental breakfast was very well stocked, so we definitely didn't go hungry! We also ate dinner in the restaurant the evening before. The pizza is delicious, decent size and done in a wood-fired oven. The only issue was that the restaurant was cold, as the main door was wide open even though it was cold and windy outside. We had to ask to have it shut, after which is became much warmer. People were still wearing their coats, though.
All in all, it was a good experience. It is very much worth it, just for the atmosphere and history around you."