We had a great experience with these guys.
My boyfriend and I took the Canyon Rim Ride on a Friday morning in January. Our group had 6 riders and a wrangler. The horses were nice and seemed appropriate for riders of all levels (although I really had to get on my horse's ass to get him going). The ride itself was amazingly beautiful -- stunning vistas of the canyon at every turn, beautiful desert landscape, etc. Although there is no trot, canter, etc on this ride (it's all walking), it was very enjoyable and the terrain was varied and challenging.
I would definitely recommend Cowboy Trail Rides to anyone looking for a great horseback riding experience in the desert. Pricing is reasonable as well.