Had the exact same experience as Jet Lag returning from Conrad to BKK on 12/23.

I didn't argure and paid 400 Baht plus toll to the airport. This was a taxi called by Conrad bell boy. It was 4:00AM was not awake enough to argue and later realized I did not receive the taxi card Conrad usually hand me for reporting any problem with the taxi driver.

Had another unhappy taxi experience for my this visit to Bangkok. I had a doctor appointment at Bumrungrad Hospital one day at 7PM. I totally under estimate the traffic and only leave Conrad hotel at 6:30PM. The traffic was really bad, constantly sat on the car for few mins not moving much. When I finally saw the Hospital sign and ask to pay and walk the driver won't let me. I thought at first he didn't understand English well and didn't catch what I mean when I explain by hand gesture of walking. After more sat idle on the car and more time past that I was so late for my appointment, I asked again how far if I walk and I really want to get off, the driver from not understand English to clearly told me that I have to pay 100 baht to get off (the meter was around 70 baht) in a threaten tone. I am not sure if passenage is obligated to reach his/her destination before he/she can get off. But not a very good experience especially for situation of needing a doctor visit.