I've used Yellow Cab in the past. The taxi always arrives much later than the estimated time. Which can be expected when you call to have it as soon as it's available.
My car is down and I had an important appointment. I called an hour and a half in advance to schedule a taxi to arrive an hour before the appointment. No matter how late the taxi would be, I'd still make it, right? Not so much.
Calling in fifteen minute intervals after it was late, I was assured it was right around the corner. Fifty-five minutes after it was supposed to arrive, still no sign of the taxi. I missed my meeting.