CF... My wife and I did this itinerary last January, and enjoyed it so much we've booked it again for this coming January! You will LOVE Windstar, and this itinerary is awesome. I've told everyone that every day of the trip we would be sitting on this gorgeous beach thinking, "There's no way it could get any better than this!" But the next day was even better!!
As for what excursions to take, it really depends on what you like to do. We're "beach people" and prefer to do things on our own, so we only booked one excursion last year - to the Baths in Virgin Gorda. The only reason we booked that one was because we really wanted to see the Baths (its a "must see"), and it was difficult to get there on your own. We were a little disappointed in the excursion, because we only had about 1 1/2 hours to explore the Baths. We're looking for an independent tour to the Baths for this year's trip. We may consider a few of the others that sounded nice, and got really good reviews from some of our cruisemates last year.
All of the other islands on this itinerary are very easy to get around on your own, which is what we did. We rented a car on St. John and St. Barths, and hiked into town in Tortola and St. Martin. In Jost, the ship will tender you in to either Great Bay or White Bay, and they will have the ship's water toys available on the beach, which makes this a very fun, relaxing, inexpensive day (no taxis or other transportation required!).
Windstar's website has descriptions for the excursions available at each of those ports. shoretrips.com is an independent site where you can book excursions outside of the cruise lines. They usually have a few more than the cruise line, and they're a bit less expensive. It really depends on what you like to do, and how comfortable you are hitting out on your own. Some people insist on only booking excursions through the cruise line for fear that if they're out on their own, and don't make it back to the ship on time they'll miss the boat! While that's true on the larger lines (if the schedule says they leave at 6:00 PM, the plank goes up at 6:00 PM!!), Windstar is more relaxed about this. They have a board near the gangway where you note if you're going ashore, and when you've come back. If it looks like someone is still ashore when the ship is ready to sail, they'll make an announcement. I'm sure they won't wait forever, but it appeared they would make every effort to make sure no one is left behind.
Sorry for the long winded reply... Maybe some of this has been helpful. Let me know if you have more specific questions, and I'll be happy to try to answer them. You're going to have a great trip!!