I'm going to be honest and tell you about experiences I've had. Good and bad. Sadly, it's mostly bad.
First bad experience, called for a cab twice because somehow they neglected to send a cab out the first time. The customer service person was rude. I had to wait an hour and a half for a cab.
Next experience, the driver neglected to tell me his credit card machine was broken BEFORE I got into the cab. So we pulled into a circle K to get change. I finally arrived at my destination and the guy had no change!! I basically got swindled from the driver.
Next experience, the guy claimed he only knew his way around Mesa. I'm pretty sure Discount Cab services all metropolitan phoenix area. To NOT know your way around PHOENIX????!!! Come on!
Next experience, the guy went about 10 miles under the speed limit. These methods only piss people off. DO NOT TRY THAT CRAP WITH PEOPLE!
Most cabs do not have GPS. If they do, its probably a scam from the drivers NOT to use it. Admittedly, some drivers are nice enough to pull out their own GPS from their phones.
Good experiences:
Despite the headaches, the cab services does get me there. And that's pretty much it. Sad, I know.
If you are planning to take a cab - always call ahead and schedule. At least an hour. It works better that way.