I don't drive any of that, but it basically works like this:
You still buy the car at full price and actually pay the full monthly payment.
Then you must stick the ads on your car and claim a "refund" each month , i guess you can call it "advertising commission" .
There's usually different "plans" based on how many km your drive (so you need to send odometer readings and such too) etc.
The ads can also change , so they will send you new stickers to put on at certain times etc.
You need to send them photos,odometer readings,timestamps etc of your car each month to get that refund.
The complaints i've read is, the companies don't always pay up and they don't always make it easy for you to upload your photos (or contact them). Kinda like working with a government agency by the sounds of things. Also the different "plans" that determines how much you can claim per month can be misleading, so i suppose read carefully what it entails.
I suppose the good part, if you eventually feel screw it, this sucks, you just tear off the adverts and stop claiming.
I suggest you go to Hellopeter.com and read the complaints of these companies. I believe the one big company doing this is called "Just Group Africa" . Do a search on hellopeter and see what people are complaining about, that's a good start.
EDIT: Start here :
http://hellopeter.com/search-reports?keyword=just+group+africa
As you can see 99% of the complaints relate to people not getting their refunds/claims.