Probably the best kept secret about taxis is that there are two rates. If you get a taxi at the door of your hotel it is more expensive than if you flag one down along the street.
...Taxis are very expensive. A ride from hotel Sheraton to Punta Cancun (where many of the nightclubs are located) is about 3 kilometers (under 2 miles), and the cost is about $4.50US if you catch the taxi along the street, or about $6 from the door of your hotel. The bus would cost less than $1.50 for two people.
...From the Hilton Cancun to Punta Cancun is about 8 kilometers (under 5 miles), and the cost is about $7US if you catch the taxi along the street, or about $9 from the door of your hotel. The bus would cost less than $1.50 for two people.
...Agree to a price before you actually get comfortable in the cab.
Taxi fares are based on a network of zones. The more zones that you cross through, the greater the cost. There are no meters in taxis throughout the Yucatan.
...Are taxis safe? Typically yes, but we really don't think it would be a good idea for a single woman to get into the taxi at 2 or 3am after leaving the clubs. We've heard stories about drivers harrassing women in this situation. When leaving the clubs, take the bus, and make sure there are other tourists on it.
...I usually pay the driver as we are heading to our destination, but it's probably best to pay when you arrive. And you'll definitely want to take a glance back as you exit the cab, to make sure you didn't drop, or forget, anything. If you leave it in a cab, the chances of getting it back are very slim.