mTAXI 1.3
It seems slow as you are focused on the app as it changes pages, however tested against website bookings it is a comparative way to book a taxi.
On the first booking you will need to enter your name and number which is stored and will be used in future, saving you taxi booking time. The iPhone app uses geo location to identify where you are. The accuracy depends on the reception levels of your phone and the warm up time.
New features are being developed for example the Taxi Rank Finder in Newcastle. There is an Automated Dial phone feature which calls the taxi company directly if you require this alternative.
This application does not guarantee that a taxi will arrive, however it will book the taxi for you. It is up to the taxi fleet to despatch a taxi to you and for the driver to accept the booking. This happens in most cases although at busy times it could take a while.