For Thai food in the Vegas area, it's fantastic. For Thai food in general, it's okay.
Despite being off the strip and in a secluded plaza, Lotus of Siam is hyped up by almost any Vegas native/frequenter who recommends Thai food to you. The head chef is a chef who formerly worked at the Wynn, so it's easy to see why people are excited about it. The place is usually packed with people, so reservations might be recommended (although we just walked in and had no problem getting seated).
The food itself isn't bad. It's chock full of flavor, but I can't say it's the best Thai food I've ever had considering each dish is in the double digits. (Not unusual for Vegas.) We ordered the Pad Thai and sea bass with drunken noodles with two Thai iced teas; everything was decent, but not worth the money, in my opinion.
Lotus of Siam is worth checking out if you're craving some Thai food while in Vegas, but keep your expectations lower if you come from an area that is familiar with Thai food.