We've been to WDW 6 times; the first time we made the rookie mistake of using what was then the Disney/Mears bus service (which turned into DME). It just didn't suit us at all and we had a bad experience with it. The next five trips, including most recently this past August, we have used Tiffany Town Car.
We LOVE Tiffany. There is a coupon on their web site (usually) -- very friendly, efficient, never a problem. Renata is our favorite driver.