I can understand the appeal of having transportation lined up in advance, but I wanted to report that we took a regular taxi from the new airport to JW Marriott (Sukhomvit), having arrived around midnight, and had no problem. There was a bit of a line at the taxi stand, but it moved very quickly - less than five minutes for us to get to the front. The fare came to 600 THB (plus the tolls? 80 THB) if I recall correctly). There were LOTS of taxis available, and the line was being managed well by several "starters".

Because there were four of us, we paid 1050 bht for an Oat SUV from the airport to the Oriental, about 400 baht for a regular taxi from Oriental to the airport (with luggage tied into the trunk!) and again 1050 bht for an Oat SUV from the airport to the Peninsula and back from the pen to the airport. They are the bright yellow stands all over the place in the new airport.