I have used Town Taxi on several occasions with varying results. Flagging a cab outside of North or South Stations, I have never had a problem with Town Taxi getting me where I needed to go. Their rates are reasonable and their drivers do not tend to run you around town as if you didn't know where you were going.
However, on the occasions that I have called Town Taxi for service going from my apartment or any other specific destination, I have had trouble getting them to show up. On one occasion, I called to confirm my time for pickup to the airport only to have the receptionist say the cab was en route. They never did show up (I waited another half hour) and I was left scrambling around at the last minute before my flight trying to find other means of transportation. This experience definitely soured my view of this company and I would be hesitant to use them if I had a choice in the future.