i love the mandarin oriental, chiangmai. and i like the spa setting. it's possible one of the most ambitious hotels i've been in when it comes to decoration and the spa is no exception.
i felt that i have had better massage elsewhere though.
so, go for the ambience and the hotel experience. the massage is ok too.
they often have terrific deals, so if u get those, u r in for a treat and an excellent value for money. if not, it can be pricey. in any case, it's worth the effort.