Si, a friend of mine does this as his full time job and seems to do really well. Has his own business with his dad I think and they sub in other drivers as needed. They have a good client base which is important.

Car wise they use Audi A8's. Switched into these from S class Mercs as they spent too much time back at the dealers going wrong.

I think if you invest in a big luxury car to do this it only pays if its being used all the time. Bit like airplanes. They dont make money sitting on the ground.