Just spent the last four days in Bangkok and - with one exception - Uber rides were much more expensive than simply taking a meter taxi. Meter taxi to BKK this morning was 295 thb on the meter; Uber wanted a flat rate of 675 thb.
All of my taxi rides in/around town were 50 thb or less on the meter; Uber consistently quoted ranges of 60-100 thb (most of these rides were between 7:00 and 8:00 pm.)
Definitely pays to compare if you have a choice in the matter.