I didn't book the tickets with transportation, but I have used Mears many times. Most of the time I'm in a van with at least one other family. I would say it takes at least 30 minutes, not up to. One of the times I went, they dropped off at Sea World on the way to Universal. It seems like I'm picked up in the 8:15 range, and I'm in the park with in a few minutes of 9:00 give or take.
Coming home is a different story. They pick you up in a bus, and you will make other stops along the way (usually off site somewhere, and then onto the Disney Resorts). I always build in 90 minutes for the ride back to my resort if I have some place to go, but it never takes that long (but over an hour is common in my experience). I usually stay at the Boardwalk, and we were the third or fourth stop coming back, including a stop at Sea World to pick up more passengers.
Overall, Mears is a reasonable and reliable service, in my opinion. Since it's usually just me or me and one other person, it's the most economical way to get to US from Disney. If you have more people, it may be worth taking a cab or car service.
