We have one - there are pros and cons but on balance I wouldn't be without it.
Some pros ...
It feels much nicer being up off the ground when you are surrounded by a herd of elephants or you know there is a snake around.
It can function as a night-ime hide in game parks.
Up higher you catch a breeze.
With practice they are easy and quick to put up and down.
It is nice to be able to leave your bedding up there rather than cluttering up the car.
Some cons ...
Even if they are easy to put up and down it is nice not to have to do so - especially when you are off on an early morning game drive.
They can catch the wind. We have had it flip closed (not with us in it!) in windy conditions.
They are heavy and it is not ideal to have weight on your roof - this is especially so if you're looking at having two.
They are very expensive. Here in the UK now you're looking at £1500. Much cheaper in SA - not sure in Kenya etc. They will be SA imports so likely to be more expensive. Remember to also cost the roof rack.
I'd agree with the above. Maybe hire one first and see how you get on.