While there's nothing wrong with the stereotypical 'dive' type Thai place with photos of Thailand & the King on the walls, it's nice to know of at least one upscale Thai place -- especially one that's not overly expensive-- and Krung Siam is it for me!
I was solo & sat at the bar. Waitstaff was very friendly, and my drink & appetizer arrived quickly, with my entree showing-up a suitable amount of time later. Ambience is kind of like a bar/lounge (which is I think how the place markets itself). They had music up somewhat loud. Not uncomfortably so, but if you're there on a date or with a group of people, it might be hard to carry on much of a conversation.
I was there on a Thursday evening at about 6PM. The place wasn't crowded at all. For this type of place plus being in Vegas, I was probably there at least an hour or two before it might start to get busy.