I think it would depend on the business. I haven't seen any studies for car wraps or graphics but I have clients who have this done and it helps in many tangible and in-tangible ways.
- People seem to see their cars everyone around town (branding)
- People will learn about your business that have never even heard of it before, resulting in new business.
- I've never seen a professional wrap done on a car that I thought to myself "I'd never use them, that's tacky / unprofessional to have a car wrap." If it was a high paid attorney - then yeah probably.
- Park them in front of clients, high traffic areas - It's as good as a billboard in some cases and much cheaper.
- You can write off the cost of the stickers/wraps but not the car ;) as a marketing expense
Main Negatives I see is if you do it for the wrong type of business. But, I think many businesses it will work well for. I tend to find tasteful wraps much better than just stickers - which seem cheap / poor. No one wants to do business with someone who isn't successful. So if you do it, do it right and get it done professionally.