Sorry to bump the thread. I can confirm that the FriendlyFarang's experience at Mae Sai was identical to my border bounce at the beginning of June.
Note: I have a one-year extension of stay (Non-B/based on employment) in a US passport and a multiple re-entry permit.
Turned up late morning on a Saturday. Got stamped out then crossed over and went to Burmese Immigration. They politely told me to sit down for my photo and an entry permit. Asked for 500 baht but countered with a clean US$10 which was accepted without any hassle. Eventually got my Burmese entry-permit only to wait around in another room to enter while they asked if this was my first time to Myanmar and what I was doing. Told them I was shopping which they didn't have a problem with.
Walked up to the scenic pagoda and eventually got a motorbike up to the casino. All slots, low-limit baccarat (200 baht minimum per hand) and one other odd card game. No craps, blackjack or roulette. Asked about golf and was told it was 550 baht for 18 holes. Ended up getting an hourly shuttle (free) back to the border, wandered a bit, picked up some beers and headed back.
Gave Burmese Immigration my entry permit, they got my passport after 2-3 minutes of looking around, then told me to wait while they crossed the street with my passport in hand. After a couple of awkward/nervous minutes, they brought me my passport with Burmese entry and exit stamps. Crossed back into Thailand. Needed to fill out a Thai entry/exit card (TM6). Very few people crossing, though there was a small group of Chinese sorting VoA's. Was politely questioned about my condo's address given the limited space on the TM6 but was correctly stamped in based on my extension. Received a quick pat down at customs and then sent on my way.
Hope this helps. Wouldn't mind doing again (possibly overnighting in Burma) but there doesn't appear much to do in Tachleik aside from shop, gamble, drink, and golf. Booze was much cheaper but no idea if the "Smirinoff" or other spirits are genuine. Everywhere takes baht so no need to worry about switching money. If you need a place to stay in Mae Sai, a single room at the Orasa Hotel 150m from the border is 800 baht for a nice single room w/ breakfast. There are cheaper options no doubt but for the proximity to the border, food etc. it was sound. Very fast wifi as well.