We just returned from the World and I would use ME again as long as it is free, but I certainly wouldn't pay for it.
When we arrived at the airport, we had to wait quite a bit of time for the bus but once on, it was pretty quick. However, for our return, we waited outside our hotel as instructed, we were out there 20 minutes early. 5 minutes after our scheduled time I asked the girl at the valet desk if the bus had arrived yet, just making sure, and she looked at the book and said no, that time pickup had not signed in and they had to sign in. I asked her if someone would announce when our bus arrived and she said the driver would do this when he came up to the desk to sign in. So we waited. Buses came and went, but nobody ever came to sign in at the desk. Finally I went up and asked another lady about it and she looked at the book and said yes, the bus had already come and gone. What? How did we miss it? We were sitting right here! I told her I had just asked the other girl and so she asked her if she remembered me. The girl looked at me and said yes and when questioned by the other lady just gave a shrug like oh well. We had a really great trip and I was just so disheartened by her attitude. We ended up being scheduled on the next bus and barely arrived at the airport in time, that extra time would have made a huge difference.
Having said all that, and I do apologize for being so long-winded, I wouldn't let one experience completely ruin it. Just know that you have to really watch for that pickup bus and as long as it remains a free service I would give it another shot.