We have been going to St. John yearly for the past 2 years, and are going again in November. Prior to St John we went to Vieques for a few years.
We rent a house on the Coral bay side... it's quiet and peaceful. Get a Jeep in St. Thomas (Usually you can get them to pick you up from the airport) and take it on the Ferry to St. John (Have patience with the ferry and don't show up 15 minutes before the ferry goes expecting to get on it. During high season expect to wait at least 2-3 ferries worth)
(Without a car of some kind, you're going to be fairly miserable getting to and from the beaches)
There's a pretty decent grocery store near the ferry on St. John, but more expensive than you're probably used to.
My recommendation is to forget eating out on the island. There's a few places to eat, but overall the food isn't anything to get excited about, and it's pretty damned expensive. Get a house with a kitchen, and bring a couple of coolers with you (checked onto the plane) with your favorite meats, and pick up other critical food items at the Cost-U-less on the way to the ferry. You'll save a ton of money, and if you're any kind of good in the kitchen you will be eating the best food on the island.
(To be fair, the Pink House is pretty good, and there's a new place just outside of Coral bay that isn't bad either. But the prices are... well, it's island pricing, mon.)
If you go during the winter and plan to actually be in the water, I recommend a wet suit. For me, it's ok for the first 30-40 minutes in the water... then I start getting cold. Even a spring suit makes a huge difference and lets me stay out for as long as I want to stay out. Also, a wet suit is better than sun block. I also recommend fins that go with booties, as the urchins and coral will eat your feet up in some of the better snorkeling spots.
Don't go to St. John expecting big night life or haute cuisine. Go because it's quiet, peaceful, and has amazing snorkeling. If you're a big spender, spend on a boat excursion and a great house with a view. Go there to unwind and forget about your worries. A week is about the minimum I'd personally want to invest in a trip, given the logistical challenges of getting down there and navigating the ferry.