We happened upon Khao San one Saturday evening without reservations and were graciously seated by the host.
Our meal of Fried Tofu, Green Curry and Charred Noodles with Stir Fried Vegetables was excellent. The service was prompt and friendly. Even though the restaurant was nearly full, it wasn't super noisy like it would have been back home in the US. The prices were quite fair.
My husband and I had a wonderful dinner at Khao San. Definitely recommend!!