Setup your account. The app automatically locates you via the iPhone GPS and provides you with the taxi providers nearest you. You touch the one of your choice. Tell the app the time you want to be picked up. The app sends you a confirmation of the scheduled pickup.
The taxi showed up on time. When it did, I got another text message letting me know the taxi was outside waiting. When I arrived at the airport, the app knew the price. I touched pay and entered the amount. The taxi driver received a confirmation number. I got an email receipt. I opted to also receive monthly email receipts.
My only gripe? The app icon is ugly.