Every time I make the trip to Las Vegas, LOS is a must. Over a number of visits, I've had the chance to sample a broad swath of their menu, and the food lives up to the mountains of accolades. The flavors are authentic Thai - you can taste and smell the fish sauce and the kaffir lime leaf and the birds eye chilis. Lunch after lunch, dinner after dinner, it's always consistently excellent.
A tip for the culinarily curious - order the funky sounding stuff, and/or anything off the northern Thai specials page. That's where the money is at LOS (if you're looking for your #2 Special sweet, peanutty, saucy pad thai with Thai iced tea, you may be not be as impressed - LOS is not exactly that kind of Thai restaurant).