We also did the Land and Sea tour and it was GREAT! You get to see all the highlights the island has to offer (touristy and non touristy) and gives you an opportunity to talk with many village people as well. One of my fav. stops was in a small fishing village where we met a young boy mending fishing nets on his grandfather"s boat. We also did a very quick photo op @ Marigot Bay. Would have LOVED to stop there! Kyriecat is spot on with her description. It is well worth the $.
As for the buses,,,,,,don't remember being uncomfortable on the bus and I was sitting right @ the back. It was very hot out that day so I'm pretty sure the bus had A/C. This was with Summit '02.