We've done it a couple of times, but not to St. Maarten. We didn't have any problems with our trades and were quite happy with the accommodations. Just called MS, told them where and when we wanted to go. They told us what resorts were available and then I spent a day or so researching them. Called back and told them which one I wanted.
I'm sure that with more popular destinations, especially depending on the time of year, it might not be quite that easy to get what you want.
You should recognize that trading is not the wisest use of your points from a financial standpoint. If you're OK with that, as we are, then trading can be a nice way to get a different vacation experience.