Vans come in 12 passenger or 15 passenger. They have either 3 or 4 rows behind the driver's seat, with 3 seats in most of the rows and 4 seats in the very back.
The 15 passenger vans are more dangerous than regular cars because they roll over fairly easily. They also have far less trunk room. A 12 passenger van is a very safe vehicle. They can be driven at full highway speeds with no issues at all, and if you rent a good one, the seats are just as comfortable as a regular car. The rows of seats are obviously a little less comfortable to most people than the very front seats for the driver and front passenger, but they're no more uncomfortable than any other row of 3 seats in a car (although they are slightly more narrow, so bigger adults may not want to sit right next to each other).
I drive a 12 passenger van for my everyday car and have for years. It took me a few minutes to figure out how big it was and how much of the road it took up when I first started driving it, but it's really no big deal at all--especially on the Interstate. It used to be that the vans that most companies rented out were Ford Econolines. They were fine, but they don't always have a lot of the luxury options that you may be used to in your regular car. Those were discontinued a few years ago, and it looks like a lot of the rental companies are moving to the Nissan NV. It only comes in 12 passenger (no 15 passenger option), but it's a nice car. Or at least, it can be. I have one, and it's not the cheapest model, so I don't know how the stripped down versions compare. It's very easy to handle, and unlike the Econoline, the engine isn't really close to the driver, so it's very quiet to drive. We drive 16 hours to WDW, and I love doing it in our van. I even tend to sit in the back seat a lot sandwiched in between car seats because we always have a baby in the family (6 sons ages 12 and under). A 16 hour trip is never exactly fun, but the car isn't more uncomfortable than any other car.
