Bangkok is the only place where I found metered taxis, and nowhere in Thailand is this so necessary as here. When all is right with the world, taxis are the most reliable point-to-point tansportation in Bangkok. They are affordable, air-conditioned (yay!), and you don't have to negotiate the price, no matter what the driver might tell you. Hustles in Thailand tend to be non-threatening to life-and-limb, but here are a couple of my experiences:
1) My girlfriend and I treated ourselves to a final, 5-star hotel night at the Shangri-La, and when we hailed a taxi, the guy drove us half a block from the swanky entrance before telling us that his meter didn't work. It wasn't until I opened the door to get out of the cab that he finally relented and turned the meter on.
2) When you arrive at the airport, TAKE A TAXI. Ignore the beautiful, neatly dressed Thai women with official badges and friendly smiles who oh-so-helpfully offer you a taxi into the city. Do you see that really long line for a metered taxi? There is a reason that those people are enduring that long line, rather than walking up up to these professional-looking car services. A laminated I.D. means NOTHING in Thailand. Just stand in line and get the taxi.
3) Just a warning about going to the Hualamphong train station (a.k.a. the Den of Hustle) - when my girlfriend and I took a cab to the station, the taxi driver asked us where we were going. We told him that we were going to take a train to Ayutthaya. The driver dropped us off at the opposite side of the station and pointed us to a 'travel office' where we could buy tickets. We knew that we were being hustled, so we paid the taxi driver, then walked around to the front entrance of the station.