The reported average time from airplane to resort drop-off is 70 minutes. This is comparable with other modes of ground transportation.
The time savings comes from not having to go to baggage claim and wait for your bags. You get sent special luggage tags, and you put them on at home. WHen you luggage arrives in Orlando, it gets pulled from the rest of the luggage, and gets sent on its way to the inside of your resort room, and should arrive there about 3 hours or so after you do. It's important to take this into account, and to allow for it by smartly packing in carry-ons, packing for your immediate needs. You don;t have to wait around for your bags to arrive, they will be brought inside your room whether you're there or not. The DME tags have barcodes, and those barcodes are scanned 5 times between airplane and the inside of your room. And even the trucks that deliver the luggage are tracked by GPS. It's a very sophisticated, tech-savvy system.
DME buses are supposed to leave the airport no longer than 15 minutes after the first guest boards the bus. The service is NOT designed to wait around until buses are full.
My two experiences with DME were great. The first time (the 4th day DME was in service) it took us 63 minutes from walking off the airplane to being dropped off at CBR, and this recent time it took us 56 minutes from walking off the airplane to being dropped off at POFQ. I can't imagine a car service getting me to my resort much quicker than 56 minutes. You have to get from the airplane to baggage claim, which won't be any quicker than about 5 minutes (including waiting for the next tram, etc.). Waiting for your bags, ALL your bags, can easily take 15 minutes. Waiting for the driver to bring the vehicle to you from the parking garage, then loading up, let's say that's accomplished in 5 minutes. Then the ride to most WDW resorts is about 30 minutes. So the quickest I could see it happening with a car service is 55 minutes (if you checked bags), and 3 months ago I did it in 56 minutes with DME.
Decide between private transportation and DME not on speed, but on level of service. If a private ride and personalized service is important for you, then you should hire a car service. But if sharing a bus with a lot of other people doesn't bother you, and paying ZERO appeals to you, then DME sounds just fine. Choose on service, because speed is comparable.