As a Londoner my experience of the black cabs that operate in the capital has been consistently good. Drivers are polite, chatty, sympathetic, amusing, helpful, trustworthy and have a brain that has memorised the a- z book of London street maps. They therefore rarely get lost. You do however, pay for the service. On the other hand, taxi services in other parts of the world can be extremely trying.
Travel books offering advice on transport in other countries, more often than not. will have advisory warnings about taxi services being in cahoots with bag snatchers/muggers/mafia. They may kidnap you, drive you to a hotel where they get commission, badger you about tours, ask for an extortionate fare on arrival then try to charge extra for luggage. From experience I can add:-
a) they get lost - if a driver looks even slightly vague when you mention the address don't get in. In Australia I often end up reading out of a Melways (street maps of Melbourne) and directing the driver to my destination.
b) they will take you to a tour agency when you ask for the bus station ("no more buses today") - insist on going to the bus station and don't pay until you are sure you are in the right place.
c) they try to charge extra for 4 people sharing - negotiate the price before you get in and stick to it.
d) If you are on a meter the driver will often take the longest route in order to bump up the price - do some research on the route to your destination.
One piece of advice books do give is to insist on a taxi meter as an effective way of preventing overcharging. I have in the past also found this to be effective. However, recently in Hanoi I discovered a new scam. Taking a taxi, on the meter, to the bus station to book a ticket to Ha Long Bay cost 50,000 dong (the long way around). On the way back I shared a taxi with three other people, on the meter again. Cost 254,000 dong for the same journey. The driver insisted that the meter was right but knowing that we had been scammed we refused to pay the full amount. After a protracted argument we ended up paying 90,000 dong.
The next day I caught another taxi to the same station to leave Hanoi. It was early and there was little traffic on the road. In the city one of the main noises you will hear 24 hours a day is horns blasting out. They will beep to let you know they are behind/driving towards/passing/about to hit you. They toot to say hello/goodbye/I'm here/I'm waiting/I'm fed up/move your ass. The length of the sound and amount of times they hit the hom depends on the message they wish to convey. So it was no surprise when my taxi driver blasted away at the traffic, human and automated, all the way to the station. What was odd though was that he hit the horn over and over on empty stretches of road. I put it down to over-enthusiasm. On reaching my destination the meter showed 84,000 dong. Another argument ensued. I still had no idea how the meter was being fixed but later fell into conversation with other travellers and was enlightened. The meter is connected to the horn and every time it is sounded more money gets added.
Today, I arrived back in Hanoi from Ha Long Bay. Taxi drivers were swarming around me as I collected my luggage. "How much to the lake" I asked. "120,000 dong" they replied. "Too much, 50,000 dong" I said. "We go on taxi meter, no problem" they promptly responded. "No taxi meter, beep beep" I retorted. They left me alone. I caught a motorbike taxi for 20,000 dong and still probably paid too much.