A few friends and I went to Qua Baths & Spa for my birthday a couple months ago. We normally do the spa at the Encore, which we absolutely love.
It is pretty pricey if you go for a spa package. They accept SpaFinder though.
Qua is a completely different experience. I always get a deep-tissue massage, and the one I got at Qua was the best I've ever had. I enjoyed the different steam rooms, and one had a eucalyptus feel to it. The Roman bath toward the front is massive and you can lay on the heated stone chairs forever. It does get crowded, so don't look forward to too much solitude or quiet. One of my friends had never been to a spa before, and he didn't want to leave when it was time for dinner.
The only thing negative I could say is that the attendants did not offer to provide a tour for newcomers. Although I don't need it, my friends mentioned they would enjoy an outdoor area to relax.