Do you manage to keep it reasonably clean?
A friend of mine who is a plasterer part time, has been for many a successfull year has a van with some magnetic signs that he had made up, thought if i do use my car i could get some made up and whack them on the car when needed.
Funnily enough though he's on his second set now as some toe rag nicked the signs of his van whilst he was doing a job, he found it funny that some thief could be driving roung advertising his plastering business for free as the signs were permanent and printed with his name and company logo and contact details!!!
Back on topic though... my only hang up about using my car is ruining the interior with dust from adhesives and grout.
Posetives are that when i do get up and running, it's a new, smart looking estate, with the magnetic signs i think it would look the business.