We took that train from Trenton NJ to Orlando, not for a cruise, but down to Disney World.
I can add a couple of things. IIRC, the auto-train added as much to the cost as renting a car would, so we just rented one and enjoyed driving "someone else's" car on our vacation.
AMTRAK used to offer like half price fare for kids 10 and under (I think they still do), this made the math work much better if you have a family with children.
It takes about as long (a little longer) than driving down yourself, but we did it for the experience of it. The kids had a blast! One of the highlights of our whole vacation.
It is not for the faint of heart. If you like long drives in cars or overnight plane rides, you'll be fine, but if you have a bad back or a medical condition, or just get claustrophobic or bored easily, you will not be happy.