I've used DME five times (May '05, Sept '05, Dec '05, Jan '06, and May '06) and will be using it again in Sept. We had good experiences every time--left the airport in a timely manner, bags in room when we get back from the parks, first drop off coming from the airport and go straight to the airport with no stops on the way back, drivers friendly and safe.
I did forget to tip the DME driver the first time I used it. I was under the impression that Disney took care of the tips, but have since learned dfferently.