One of the advantages of taxi meters is that they provide an accurate measure of what the fare should be. However, that doesn't mean that there aren't ways for taxi drivers to take advantage of their passengers. While the vast majority of cab drivers would never take advantage of a passenger, it pays to be on your guard in case you encounter one of the few drivers who would.
A common taxi driver scam is to get more money from a fare by taking the long way. While that sometimes means taking a less-than-direct route, in egregious cases it can mean driving passengers around in circles. This type of scam usually only works on people who are unfamiliar with the city they're in. If you're traveling, keep a map handy and know the basics of the most direct route for where you want to go. You can even download smartphone apps that calculate the most direct route and let you know what the fare should be.
While taking the long way is a low-tech scam, drivers can also take advantage of passengers by adding surcharges or changing the fare type to one that costs more. Remember how fares that take the cab into Westchester and Nassau Counties from New York City cost double the rate of travel within the city? Fares within the city are Fare Code 1; drivers set the fare code when they pick up passengers. In 2010, New York City officials found that cab drivers had been charging the Nassau and Westchester fare rates for fares in the city by entering Fare Code 4, which tells the meter to calculate the fare at twice the rate for travel within the city. You can protect yourself from scam like this by knowing the fare codes for the city you're in and making sure the driver is using the right one. Fare codes should be posted in the cab. If you feel like the driver is using the wrong code, get their license information (which should also be posted in the cab) and report them to their city's taxi commission.
Taxi meters use objective measures to protect drivers and passengers. As long as you pay attention to the meter and know your rights, there's no reason to worry that your next cab ride will be anything less than wonderful -- assuming, that is, you can find one.