My new favorite spa in Vegas- and I've been to them all! Amazing Korean style spa. Lots of options for bathing, soaking, steaming and sauna-ing. Self-directed yoga room! Korean food cafe! You can definitely spend half a day here just chilling, and they're not trying to rush you out.
I had a 60 minute deep tissue massage with Ben ($90) and it was one of the best massages of my life- and again, I've had many many massages! He was not afraid to massage everywhere but was totally professional and respectful even as he worked on big painful knots in my butt.
If your treatment is $80+ you get day spa admission included so its really a lot of value. With taxi from city center (aria/vdara) at $40 round trip, $90 + 20 tip for massage, and $15 for food, I spent about $145 (splitting taxi with friend) and didn't regret a single dollar.
We went on an early Saturday morning (9:15) and had the place mostly to ourselves. It was heavenly.