The roof top campers are big in the off-road overlanding and Jeeping groups. Seen several of them in action. They are basically a great way of not sleeping on the ground in a tent. They open up very quickly and you will not wake up to a rock in your back on the ground like good old fashioned tent camping. I know several people with them. This eliminates the hassle of finding nonrocky levelish ground for a tent. And the setup is fast!
They are really not comp. to truck campers as most of them are guys doing 'real' offroading that a fullsize truck plus TC would not dare go down.