I live here almost cash-free. I carry a float of 200 Baht for small purchases 7-11, taxis or street vendors.

Everything else goes on plastic. TOPS supermarket and Big C as well as Tesco take Visa/Master/Amex.

You can even top up your subway or skytrain pass using Visa/Master/Amex (subject to topping up a minimum of 300 baht per transaction).

Most of my bills are auto-debited to a credit card. Thailand is very plastic-friendly indeed, at least for Visa/Master. Amex is a bit more a case of YMMV and of course a lot of the genuine Thai experiences will be cash-only. (Some old lady's hole in the wall curry shop won't take cards as you can imagine).

Overall you will find card acceptance quite solid.