I had ressies with Mears last summer for two off site (off WDW property) locations.
1. Belz Mall - as stated above, I had made ressies in advance and even confirmed them! The morning came for Mears to pick us up; we waited 1/2 hour, having seen numerous Mears shuttles come and go. I asked each Mears driver that came through if they were there for us - going to Belz - none were. But several called in on their radios to report we were waiting. Another 1/2 hour passes and still no pick up! I went inside at BW and a very kind CM called Mears for me. He explained to them that we'd been waiting an hour for a scheduled pick up. They said they would send someone right away. Yet another 1/2 hour later - still no pick up!!! The kind CM at BW got someone to drive us over to Belz in the BW van. We did take a Mears shuttle back and they didn't charge us. But what bad business!
2. Our second off site trip was to Sea World. Again, we had ressies and I even confirmed them the day before. We had an early pick up which came pretty much on time (just a few minutes late, which was fine). But the afternoon pick up was awful. Again, they were over an hour late. Many hot, tired people were waiting impatiently.
On the trip back from sea World, it became apparent that Mears is not very well organized. I could hear on their radio that the dispatcher would repeatedly call for anyone in the area of _______(insert hotels, theme parks, etc.) It was apparent that they'd schedule pick ups, but when the time came for the pick ups, they just get on the radio and try to find someone cruising around who's in the general area and can pick up the poor souls waiting for a late shuttle. It didn't seem to matter that you had a scheduled time - it just seemed to depend on who was in the general vicinity within a couple hours of your pick up time.
Sorry to go on so long, but it was a couple of really bad experiences that I'd prefer not to repeat if there are any other options!
Again, if anyone knows of other shuttle services, please let me know.