When you go to Myanmar, take plenty of US$$ - including extra for potential emergency - which happened to us. We read that the Myanmar Mayflower bank has ATM service, but did not see any of these branches. In this event, with the exception of some large hotels, there are also no credit card facilities. If you run out of cash, you will be forced to find one of these hotels, and accept a fairly hefty fee, of 30%+ for getting a cash advance - in kyat.
In Yangon, its very easy to find money changers - which is still supposed to provide the best exchange. Some are furtive fellows on street corners, but mostly at the Bogyoke Aung San Market, which is the large tourist market in the city - also known by the British name, Scott Market.
There are plenty of changers standing around in the market, who will discretely ask if you want "money change"? You then negotiate the rate - useful to get a few quotes. Then you can exchange a few US notes for a huge wad of kyat! As with some other SE Asian countries, you may get a better exchange for crisp new notes.
Some guesthouses will insist on $US anyway, and you can opt to pay for some things at larger shops or restaurants, with $$. However, business is predominantly done in kyat - and change is always in the local currency.
It is wise to count the money to ensure that you are given the correct amount. It is a big wad, and you can be easily deceived. First time changers! We were walking through the market, approached by a changer, who had the whole subterfuge scenario persona worked out! Most of the changers were somewhat more relaxed and open!