I spent a month in St. Vincent from the end of April to the end of May, 2006. Overall it was a great experience. My team and I filled our days with working with local churches, helping them with construction and other projects.
If you're thinking of traveling to St. Vincent, I would recommend it. However, be prepared for very hot weather. I got sunburned a few times while there, and if your accommodation doesn't have air conditioning, you may have a problem.
Young Island is by far the best St. Vincent has to offer in luxury stays, however it is somewhat expensive.
St. Vincent has an active volcano on the north side of the island, which is available for the energetic types to tour and climb up.
I like to think of St. Vincent as an undiscovered island. You won't see the amount of tourists that frequent nearby islands such as Barbados. In fact, when we were about 10 miles inland, there were several children that told us that they had never seen white people in real life before.
If reliable transportation is important to you, rent a car. Busses are cheap, but crowded and don't run on a schedule.