Hi! We did the Segway excursion on St. Maarten on November 30, 2006, and had a lot of fun with it. We are in our early 60's. Our adult children were with us and thought it was the best excursion that we did. Of course, because the eight of us were together, it was comical and fun. No one tipped over or ran into anything or anyone! Except the tour guide--she ran into a garbage can!
One suggestion--if you decide to do this, do the early morning excursion. We were on RCCL and booked it on the ship and went on the 8:00 AM (or something like that!) excursion. It was nice because there was no one on the boardwalk and we could move around without tourists being in the way. It wouldn't be nearly as much fun if you had to dodge people. We didn't go far on them, just from one end of the boardwalk to the other end and on the beach a little, so sightseeing wasn't an issue. I would agree with Lil' Lori that you couldn't sightsee and drive one.
When we were done with the Segways, we took a van to Marigot and went shopping. The bazaar at Marigot has many unique items and we all enjoyed that, too.