Prior or after a spa treatment only. In fact I like to do an early spa treatment and spend most of the day there, its a very relaxing area. We left only because we got hungry for lunch and later heard you can order food in from there. There are also some small snacks, and of cours spa waters.
I've also done the Rainforest Room on the Wish. They are different - cruise ship thermal spas and land based spa gardens are kind of different species of the same animal? But if you like one you are likely to enjoy the other. There are several different hot/cold pools and water features, a reflexology walk, sauna and steam room, indoor and outdoor loungers. Its quiet back there, even quieter than the adult pool.
If you are a spa person and can afford it, Aulani's spa is definitely worth it. Its one of the best.