I have the Boab Maggiolina roof tent on my Land Rover 90.
The bad points are :-
Its a real pain to bolt it to the roof bars, which requires a friend to give me a hand to lift it on.
The Landrover is just a tad tall for the wife to help, so one on the car I leave it there for the season.
It can be difficult if its too wet, as there is no where to put the wet clothes or boots.
Everything has to go inside at the head or foot of the tent.
If you stay on a muddy site, then the mess comes in with you.
There is no where to sit and eat if it rains.
Getting up in the night for the loo is a real pain due to the ladder, especially if you have to climb over someone to get out.
There is no shield from the rain when you are getting in the tent, which can soak the bedding if your not quick in bad weather.
The cost of the tent and the roof bars is a huge sum of money.
Reduced fuel economy.
The good points are :-
On the plus side you can turn up late at a site, as the time to get ready is under a minute.
You save space in the car, as its all on the roof.
You can store your bedding and then leave it in the tent.
You can move on each day to a new site, with out the pain of having to take down a tent ( under 5 minutes to stow away ).
You dont need a torch to wind it up if its dark.
Its warmer than a ground tent, although in the morning it warms up quicker on the outside than it does inside due to the insulation. You just have to get up earlier
Everyone wants to talk to you about the tent, the moment you set it up.
If the tent is wet, its not a problem to dry it out when you get home as you just wind it back up.
There are no issues with pegs in sand or stoney soil.
It survived a sand storm, when ground tents around us were destroyed.
Summary
I bought my tent to do an overland trip, although in hindsight I could have got away with a small dome tent.
The only time that a dome tent wouldn't have been OK, was when we were in a sand storm.
One night out of 3 weeks would have been an acceptable trade off.
I am happy with what I have, and I use it each year for all our camping trips through out the season.
Having said that, I could have saved a huge amount of money and just made do with a normal tent.
The 90 with the roof tent up
YouTube - Landrover Tent