I used MearsConnect from Bay Lake Tower to the airport on my last trip in early January, and did not have an issue with booking and did not have a problem getting to the airport.
However, I will say I overheard the radio chatter while on the shuttle and it sounded crazy. A whole bunch of people not showing up, and the dispatcher pleading with drivers to update their devices with passengers. In my case, I got on a shuttle about 20 minutes ahead of time because one just happened to be there, but the driver did not scan, nor even look at my confirmation. He said something to their dispatcher, but I have no clue how they would have figured out I was already on my way. So, I'm sure a little bit later another driver reported me as a no show. I'll probably use it again for a trip in early April, so will be interesting to see if they modify how it is actually run.