I have avoided them where I could on my past trip there for reasons I described before. I have used MRT and BTS instead and where I had to I used Hotel shuttles and Airport Limo Services. However that does get expensive at times.
Take a trip to Ayutthaya, for instance. Just yesterday I saw a tourist couple getting in a taxi for a trip to Ayutthaya at a price of 2500 Baht. That will most likely not be a full day trip and they will for sure end up in shopping places and overpriced restaurants, who make deals with the driver. My hotel shuttle charged only 2000 Baht and I am sure I could have negotiated them further down.
But for my trip to Ayutthaya this time, I just needed to go up to meet friends and was not sure how long it would take. So I flagged a taxi down and surely the first one stopped and took me. Meter was on and we went to Wang Noi district in Ayutthaya. Mind you, I had to tell the driver where to go, since he was not familiar with my destination, though it is a renowned landmark. Arriving, the meter said 360 Baht. Now that is a difference to 1500 or 2000 Baht!! So, I got lucky this time.
Going back was a totally different story. No taxi was available and when we called them no taxi wanted to go to Bangkok. It was still only early afternoon, but they feared not to get a customer paying for the way back. And of course 'traffic' was another reason why they did not want to go into Bangkok. Here is a surprise! A taxi which wants to avoid traffic!!
I finally ended up with a taxi from Wang Noi to the outskirts of Bangkok, a mini bus to the center of the old town, a ferry across the river from where I took a taxi to the sky train, which brought me to the subway.The combined price for it, however was barely a tenth of the Hotel's shuttle.
Laws, like in Germany where a taxi has to take you to your destination do not exist here.
Finally I am back in Bangkok. It's New Year and the city is empty. No traffic but also fewer taxis. They all went home to visit their families. The few who didn't go were visibly and acoustically outside my hotel with the speakers turned up. They were sitting around their cars having a good time and asked for customers. But they were so drunk that it would have been like playing Russian Roulette to take them. So, thank god for sky train, subway and my own feet!