The spa is very nice, but not nearly as nice Amanpuri or Trisara. I think the main problem is that the treatment rooms lack the expansive feel and nice views that you get at either of those resorts. I think one of the treatment rooms has a nice view of the lake, but the ones I saw had trees and shrubs growing right in front of the front of the room so there's no sense of openness and it felt a little dark. It's too bad, because with the setting they have it certainly would have been possible to do a better job. The hydrotherapy rooms on the other hand look out onto the lake and are very attractive.
Treatment Room
The therapists are very good, but in my experience Vietnam does not really have the same long history of massage treatments that Thailand does so I would have to rate the quality not as good as Amanpuri or a lot of other Thai resorts. That said, it was much better than what I've experienced elsewhere in Vietnam (Metropole, Park Hyatt, Nam Hai).
I've also attached a picture of the gym with great views of the lake which is very, very nice. The only problem is that my frame of reference is the gym at Amanpuri and that totally spoils you for every other gym anywhere for the rest of your life.
Gym
There is a separate Pilates studio in the spa compound and that is very nice as well.