I have friends at Walen who have given me mixed reviews about the school. I'm with Bold as Love and would highly recommend AUA. In 1.5 years time I've gone from knowing absolutely 0 Thai to verbal fluency with a good accent and grasp for tones (at least for a native English speaker with no other tonal language experience), and am approaching basic fluency in reading and writing. It's in unusual system there (search for my other AUA related posts or read my blog at
Dan's Adventures in Bangkok look at the Attempting to Learn Thai posts) but it really works and gives you a natural sense for the language.