If you enjoy lengthy history podcasts, you know Dan Carlin. Years ago he did a postcast on the dark ages and the vikings. One of the questions he discussed was how it came that war like peoples like the Franks or the Vikings could ever become Christian, supposedly a religion of peace and non violence. The answer is that they became a different type of Christian, a warrior type of Christianity.
Mont St Michel was built in Normandie, Normans being Northern Men, i.e. Vikings. It's not a coincidence that they chose this patron as Michael leads God's armies against satan.
The monastery is what you expect. The views are stunning, the site being built on top of a hill in the flood plains that mark the border between the Bretagne and Normandie. Admittedly, I found the inside of the abbey rather empty and a bit underwhelming. But the surrounding town (if it wasn't for the endless flow of tourists) was a pleasant surprise. Due to heavy rains, though, it was hard to get a nice picture as I was fearing for the safety of my camera.
Getting There
There is a direct bus connection from Rennes. At 30€ it's fairly pricey, but other public transport options (as usual in France) are poor and more complicated. I bought my bus ticket in advance online at oui sncf. The bus did fill up. The bus stop is the long distance bus station next to the Rennes train station and is well sign posted. Note: The name on the ticket has to match the name of the holder. An Asian couple had bought two tickets under the same name twice and the bus driver refused passage to one of them. They had to buy a new ticket.
In St. Michel, the bus drops you off at the shuttle parking lot. You can either take a free shuttle or walk. With waiting times for the shuttle and considering that you get the best views of Mont St Michel from the bridge anyhow, I would skip the navette and walk. Even if you plan to take the shuttle you are better served going to the 2nd station than waiting at the start station.
In the afternoon I returned to Rennes by shuttle. From there you are well connected to most parts of France. I continued to Caen/Le Havre by bus. And if you are wondering and not acquainted with the peculiarities of French public transport: No matter where you are in France, it's always easier to travel to Paris than to the next major city in the neighboring region. In this case there were no trains and just 1-2 buses in the afternoon to Caen.