I've been on two CTNs, both two nights, and both out of Boston.
One was a charter from a local travel agency (not the one I use) in 2002, on Norwegian Majesty. It was expensive for a weekend, and included all drinks, including alcoholic ones. I'm not a big drinker, but it was so nice to order my usual club soda with lime in the show lounge and not have to sign for it. This particular cruise was one that was fit in between a drydock and the start of the Bermuda season.
Our other one was in August 2000 on Premier's SeaBreeze, my favorite ship. During that summer, Premier ran them regularly, alternating with 5-night cruises to Atlantic Canada. This was made via regular booking, and was exactly like having a sea day on a typical cruise. We had a fabulous time, and actually rebooked another weekend CTN for the next month. But, the line went out of business just one day before we were to board.
I've considered a couple from Carnival out of New York, but something always came up that prevented me from going. We also looked at a 4-night CTN on Queen Mary 2 from New York, but the cost turned us off to that one right away.