I go there regularly from work and have never been disappointed. Food is amazing (I love the coconut rice and lunch time menu), and portions are large and really good value. Service is friendly and quick too.
Its quite small, but its always busy when I get there which I think is a good sign, too. Definately recommend it!
Mangosteen serve a variety of authentic Thai and vegetarian dishes at their restaurant and can cater for private parties of up to 20 guests upon request. A takeaway menu is also provided.