Hi there,
My husband and I just returned from a trip to Paris that included a side trip to MSM.
I think staying on the island was absolutely worth it. You do have to adjust your expectations and know you are staying on a a rock not truly meant for habitation. So honestly for me, I was impressed with our accommodations even though not fancy. My husband had greater expectations and was miffed about the nonfunctional wifi. That did not bother me though because I was not there for wifi. There was cable tv- all in french- but again. I wasn't there for the tv either.
We stayed at the La Vielle Auberge https://www.lavieilleauberge-montsaintmichel.com/fr/. The rates were fair because we went in the off season. Rates may be higher if you're going in May. But will definitely be higher on the island than not. I picked a room with a view of the bay. It had a huge balcony. I could've thrown a party on it. The sunrise was great in the morning. Only caveat with staying on the island is the abbey bells. They tolled all night long and excessively at 7am. The room itself thought was great- of course small and hallway outside musty- but you're in a bay on a rock. Who cares. It's amazing. The bed was super comfortable. We also had dinner in the hotel restaurant- Auberge St. Pierre? It was decent- I actually had a great lamb dish.
I can't stress enough how difficult the terrain will be so pack light. We left our baggage at our hotel in Paris and only took backpacks with us to MSM. It's uneven and stairs, stairs, stairs...The main road was cut off for construction when we were there so we had to walk through narrow alleyways to go around.
The Abbey itself is amazing. Oh and do make sure to check the official website for the tide schedule. I had a great experience considering the tide did not come in when we were there. So I'm sure it's extra magical when it does.