I use them all the time because there are no alternatives at a reasonable rate. They are almost always late, so don't book a shuttle for 12:30 and expect to be in Tucson by 2:30 as advertised. It will be more like 3:30 or even later. They really push for adding gratuity but their service is just standard, nothing impressive. I'm sitting at their waiting area in terminal 4 now (1:15pm) waiting for the 12:30 shuttle to Tucson.