You used to be able to get a day pass, like Qua, w/o being a guest of the hotel, not anymore. If you want entrance, you gotta get a service which comes w/ spa entrance. Having said that, it's worth paying for. I usually get a massage from Mike who does the best Deep Tissue massage in Vegas!
The waterfall hot tubs are always relaxing and I love the cold plunge after a long steam. It's always clean, quiet, and the staff is attentive but never overbearing or intrusive.
I had a late flight on a recent visit and it was a great way to waste an afternoon away after a heavy loss of gambling.