Mark Wiens says "the tom yum goong (ต้มยำกุ้ง), which is the dish they are most well known for, is something truly mind blowing."
I arrived at noon on a public holiday and got a seat immediately, then only had to wait 10 minutes for the dish to arrive. But by the time I left, there was a queue to get a seat. It's definitely a very famous and popular place. I couldn't see any signs for either the name of the place or even what they sell (in English at least). I also didn't see any menus but maybe I didn't look in the right place.
Anyway, the Tom Yum Goong (the famous Tom Yum soup with shrimp) is delicious and a decent size. It's fairly spicy but it was easy to eat. The only problem was that it was a really hot day and the chillies just added to it and I was sweating so much that a nice Thai man sitting opposite, ordered an iced drink for me.
Definitely worth a visit, especially as it's so close to Khaosan Road.