Mont-Saint-Michel is impressive and truly one of the symbols of France. It seems to me that many people have a problem of public transportation for visiting this place; the easiest and quickest way (that you can make it as a day trip from Paris) is take TGV to Rennes, a lovely capital city of Brittany and then take an hourly special shuttle bus to Mont-Saint-Michel which parks in front of the train station. This bus will take you to the main gate of the island. SNCF also provides a special train to Mont-Saint-Michel in Summer.

One more tip if you take the last bus back to Rennes in the evening, you will have a chance to see the famous tide of this region which is one of the quickest in this world and see Mont-Saint-Michel becomes an island again