Just a quick note to share my experience with Uber in Bangkok over the last few days - I guess a took Uber 4 or 5 times including the trip to and from BKK airport.
One thing all of these rides had in common is that the drivers spoke absolutely no English. This was however not an issue except for our last ride back to BKK (see more on this below).
In all instances I took the standard uber vehicle which was usually a standard size toyota sedan. Once we had a US style pick up truck, which was good for luggage.
In terms of fares, within town from Phloen chit until the Mandarin Oriental was 120 baht. Airport to Phloen chit and back to BKK for 350 and 375 baht respectively.
Our last driver though did not speak a single word of English - this was not great as I had already been waiting for the car for a while with no luck. I did send a couple of sms (in English) but no response. As I was not in a hurry, I waited until he made an u-turn as the missed the right exit the first time.
We also took a regular taxi once and we were lucky that the driver took is through the right route - not sure if it helped the fact that we had the phone in our hands as if following the itinerary from google maps. Anyway, this ride went well too but I was running low ob Thai cash hence I preferred to pay via Uber with Amex.