I went to Pai last week and was quite impressed with the quality the meals. I've been to Thailand a number of times, and Pai's is definitely authentic. Making it even better, the prices are very reasonable and the portion sizes are large, which explains why it was packed on a Tuesday night at 8:30p I had the Gaeng Hunglay with ox tail, which was perfectly braised in a tasty curry sauce. As for the ambiance, it is casual although there is an eclectic mix of hipsters and suits.
If I had to ding them a point - and this may be petty - it would be on the variety of the mains and the lack of off-menu specials. Otherwise, I definitely recommend Pai - I'll be going back there again.