My husband and I took a boat from St. Maarten to St. Barts almost 4 years ago (on our honeymoon) and it was AWFUL!!! The seas didn't look bad and the weather was beatiful (it was mid-late December), but it was the worst thing I have ever done in my life! Coming back from St. Barts to St. Maarten wasn't as bad (we were going with the wind), but I don't think I would ever do it again. We were all soaken wet and I would say 75% of the people were throwing up. Once you get to St. Barts, though, it's beautiful! But I would advise against it, especially if you are the least bit prone to sea-sickness!
I concur with the above. I have not been personally on the ferry but I have talked to people coming off of it plus reading trip reports. This 15 mile trip can get a littl e rough. If you like roller coaster you should take it but if you are prone to sea sickness I would stay away.
The alternative is flying into St. Barts. I don't know if you are planning a day trip or not but I believe flying in offers the most time on the island. I know flying in to Saba offers more time and I'm pretty sure it is the same for St. Barts. The flight in is pretty unique because the pilot flies through a cut in a hill and dives to the runway. It's pretty fun to watch. I'll be going back probably next November and I'm planning to fly into St. Barts and Saba.
Is it worth it? I don't know. If you have never been, you should go. I really didn't like it but that is just me. It was pretty neat the first time but I've been there four times and it's just there. Than again, I hated Paris and loved Norway, so what does that tell ya.
If you have the time do it. If not, just go to the beach and let sand gravity take you away. I hope this has helped.