Our bus actually broke down!
We took a Gray Line tour to the South Rim of the Grand Canyon Oct 24, 2009.
Just as the bus got into the Grand Canyon national park, warning lights started flashing and beeping - the bus had an air pressure leak and we could not go on. A driver from a passing bus (a different bus company) let our passengers board his bus and drove us up to the lookout. We enjoyed the view for a while and then were told that due to liability, this second bus company could not take us back to Vegas. We were told Gray Line was sending a new bus from Vegas - this means we would have to wait 5 hours for it to show up! It was already 3pm and getting cold.
Luckily a third driver was willing to drive some of us back to Vegas for cash (we paid $40) - there wasn't room for everyone. Not sure what happened to those who didn't get on this bus, but I would love to hear what happened to them and what time they got back to Vegas (probably 2am).
I've called the company seeking a refund (at least partial) but was told I am not getting one since we got back to Vegas. I wish we had read the reviews before booking.
In hindsight I can see why the Gray Line bus had only 14 passengers and the other company's buses were full or near full (80 passengers).