We have chartered many times in the USVI and BVI and this year we choose a charter in St Martin. As pointed out previously, very different from the BVI charter scene. While the beach bar in every anchorage in the BVI has led to some great vacations and memories, so can much quieter anchorages with a much different atmosphere ashore that the St Martin area offers. You will much more often be in the company of those staying ashore or with cruisers rather than other charterers.
While Sunsail and Moorings both have a presence there, it is on a much smaller scale. Well run and a little more personal attention I found.
One of the cautions is you are checking in/out of countries as you move between countries, often taking 1 - 2 hours to complete the process. So you want to allow a lot of extra time for the formalities, limit the number of separate countries that you visit, or plan your itinerary to optimize your check ins/outs.
We did enjoy the different ambience that is offered by visiting the different islands /sides of the island.
Tintamare did have moorings, the same as the other poster mentioned, Sunsail cautioned using them as some are not well maintained. Regardless of using the moorings or not, there is a park fee payable to go to any of the park anchorages. Easiest to pay at the charter office.

- Dave