I've used Desert Cab a few times with mixed results. The general wait time is 20-25 minutes for a cab, but if you're lucky, a cab will come within 10 minutes.
One time I called for a cab around 2PM. Was told the standard wait time. 2:30 rolls around and I call DC. The dispatcher said there was situation in Summerlin and told me to wait 30 minutes. Okay.... I was on the east side of town - were ALL of their cabs over there in Summerlin?? I cancelled the cab and called another company.
Another time I called for a ride, the cabbie didn't know the directions and asked me to navigate. They don't have GPS? Maybe a lot of their cabbies are all just used to taking tourists to the Strip and the Airport, but not other places in town.