We use the Autotrain quite a bit. Let me start with a disclaimer: I love to take the train, so I'm pretty biased in favor of train travel. But besides that, we like it. We travel mainly in off season, so in terms of what it's like one a crowded train, can't advise. We've gone coach and with a sleeping car.
If you put out the money for a sleeper-- we've done the family sleeper and on the last one the family sleeper and another roomette since there were six of us- you get your private room which is turned down at night at the time you specifiy, then turned back into regular seats when you go to breakfast. Dinner is better for sleeper passengers. Breakfast is the same.
When going coach-- we've always had room to stretch out. There is now a plug in every set of seats, so you can bring portable DVD, laptop, etc and not have to depend on batteries. The seats are big, they recline. They give you a pillow and blanket but we always bring our own. Food is nice-- nothing great. There are free snacks in the lounge (cookies, party mix, fruit), water and hot drinks are also free. There's also free wine at dinner.
They play a movie in the lounge twice a night.
I'm pretty sure the box on the SUV wouldn't be allowed.
We've been late once, usually are on time or early. We've had mixed results with how long it took them to get our van back to us. Once, we were the second car off, once, the last (but that was on the way home so no big deal).
We usually drive overnight on the way down, two days on the way back, and I much prefer the train to all the driving. It's more relaxing.