We took the U2FT in July 2001 when we were last in St Maarten and the Atlantis Sub in St Thomas on the same trip.
The Atlantis was pretty good IMHO. DS 9 & DD 4 both liked the trip out and back (about 20 minutes each way) and the about 45 minutes in the Sub. We got down to about 85 feet and the water was very clear. We had a great narrator and he was very knowledgeable. While we did not see an abundance of fish (I guess they did not know their Castmember cues) the whole experience was interesting.
I would do it at least once.
As to the U2FT it was interesting for what it was. They take you on a circuit around the island and point out all the sites. The buses are lined up at the end of the pier and they bring extras if needed.
You stop twice. Once at a high point so you can take photos of both the Dutch & French sides and the again for about 45 minutes in Marigot on the french side for shopping.
Then they will either drop you at Philipsburg for shopping or take you back to the pier.
The kids liked it and we felt it was ok in order to get an overview of the island.
We're going on the Magic in May and this time we're taking tha St John tour and renting a car in St Maarten.
We want to go to Orient beach and spend more time in Marigot.