Never again! I was supposed to be picked up at 6:15 at my hotel. The bus never came. I called the dispatcher and the bus driver said I missed the bus and he wouldn't be coming back. This was not true. The dispatcher told me to take a cab to the meeting point and I would be refunded - we'll see if I'm actually provided a refund. The meeting point was a chaotic mess, and I realized I was on only one of 11 packed buses. If you take thís tour, be prepared to be just one of ~600 travelers headed to the Canyon - and all 11 buses made the same stops, so restroom lines were a nightmare and the lunch buffet took 30 minutes to get through because there are only two small buffet service areas. The staff at the lunch restaurant were quite rude and inefficient, but that's not GCT's fault.
Finally, the bus is cramped (much smaller than an airplane seat). With stops, lunch, and traffic getting into the park, expect a 7.5 hour trip from your hotel to the park and then another 5+ hour journey back to your hotel. Also, I was not dropped off at my hotel but rather across the street and one long block up.
Do yourself a favor and do not take the day trip - maybe one of GCT's VIP tours are easier on the nerves.