Driving with our Tentbox is smooth. We comfortably cruise at 70mph. Just a little wind noise and a slight increase in fuel consumption (3L/km or 0.8 mpg).
It has no height restrictions, which is great for parking.
It's easy to put together. It easily opens and closes, although I needed a bit of time to get used to the ladder. It's not clear how to lock it and I'm always scared to pinch my finders.
It's big! We have so much space, it's amazing. It's a kind size bed so everyone can spread without bothering anyone.
I had the opportunity to check out various setups now. Inflatable tents caught my eye as easy to put up but still look annoying to pack. I've tried various inflatable mattresses over time, and even the top-tier ones can't compare to a real memory foam mattress. So, to match the comfort of the Tentbox, you'd also need to haul a large mattress in and out each time.
The Tentbox is overhanging from the car. We use it as an awning and it works really well.