I have been living on the island for 10 years now. When we bought the house, I did not do enough research beforehand and found out that we could not live in the house year round without a residency permit. Although, my intention was to spend winters in Aruba and summers in Mass. The first permit took longer because you need more documentation, ie good conduct, tb test, hiv test, apostle birth and marriage certificates, proof of income. Permits need to be renewed yearly. However, the renewal process is quicker. I am on my 10th permit. Next year, I will receive a permanent residency permit.
Medical would be available to you as a resident. As Andrea said, you might still want to purchase medical insurance. Luckily, I have not had to see a doctor on the island, so I don't know much about the medical care.
I do travel back to the states for a few weeks in the summer months. This is when I do all my shopping. It is the one of the things that I miss. I also miss the seafood from NE. My first stop is usually for lobster. I think that if I had grandchildren, I might make more frequent trips back. Also, it is difficult to get work done on the island. My husband gets frustrated running from store to store trying to find a part to fix something. We need a Home Depot or Lowes.
All in all I love living on the island, especially in the winter. For some, it might be too small, but I keep pretty busy.