I am one of those guests who did not have a good experience with ME and have never used it since - not even the ride back to the airport that first time.
It seems that people who have bad experiences with ME have VERY BAD experiences with ME and that's why you'll hear some of us vowing to stay away, regardless of the savings.
If time is not a huge factor for you, I'd still say to try it with the following caveats:
Be sure that your entire group is comfortable waiting. If you have one of those "We're here, we're here, let's get going!!!!!!!!!" type folks in your group, ME may not be for you. (That's me, by the way...)
If you are traveling during Pop Warner week, dance and cheer nationals or any other very large event consisting of mostly high school students, skip ME. They tend to arrive in large, large groups and back up the lines quite a bit.
Make sure you have everything you will need for a full 24-36 hours in your carry-on bag. Luggage delivery policies can vary by resort and, depending on your flight time, your luggage can be delayed until the next morning if anything goes wrong - even things completely beyond your control.
Since I'm so obviously venting, I'll give you the story:
In my case, we had a long wait (more than 45 min) for a bus, and we were at the front of the line. That was frustrating. Then we had a luggage delay - a very long one. When the front desk gave us a voucher for the hotel gift shop, they had run out of pajamas of any kind since so many people had bought them with their already issued vouchers. Not fun. The front desk later billed that voucher cost to our room - it took 3 visits to the front desk to fix this on check-out day. Extremely not fun. And then, to add salt to the wound, the ME phone folks blamed the airline - which would have been okay, except that I called them, too. They talked to the folks on the ground who were able to identify the missing bag without any description from me (it was the only one there) and said that it had been waiting for Disney for more than 10 hours. Insulting to us and the airline - and still, no fun.
Since that trip last year, I've never been willing to try again - not in the last 3 trips and not in the next 2 we have planned.
And don't fool yourself into thinking that ME is "free." Disney is not foolish, especially with things that cost them money. They know that if you skip renting a car, you'll stay on site for pretty much everything, including high-priced groceries, movie tickets and restaurants.
Jennifer
Intimate Wedding 12/11/06
Wedding Pavillion
We got Tiffany Towncar for our honeymoon arrival. The very idea of leaving the "getting of my bags" to other people just freaks me out! I"m sure it's a great experience for those who use it, but I'm going to be very glad walking right to my car with my bags in hand and being driven to my resort with no delay! I'd be obsessing about my bags and when they arrive and it would make my DF nuts! Also, Tiffany Towncar will do a 30 min stop at a grocery store on your way to the resort so you can pick up so groceries without having to outrageous resort prices! WooHOO!!! 
