I really like the W brand as a whole, but the W Bangkok is a let down. I would not go there on an important holiday. If you're looking for a good use of points look at the SGS or the Meridien.
As others have suggested though, Bangkok is perhaps the world's best value for great 5* hotels. The Siam sounds amazing (not been there yet), the Peninsula is very well regarded by many (slightly meh), and at slightly higher cost the Mandarin Oriental really is remarkable.