Spa Mandalay is one of the better spas I've experienced. I go to Vegas several times a year and always have a great time there. The workout facilities are very good for a hotel and the spa itself is great. It's not the largest on the strip - I believe that distinction belongs to Caesar's now - but it's big enough to feel spacious and not crowded.
The front desk staff is terrific. The spa attendants are courteous and they don't hover, which I appreciate. They're just as happy to watch sports in the lounge. Fine by me, I know where to find them if I need something. My big complaint there is that they don't keep up with stocking iced washcloths and chilled water. I know that's no easy task when it's busy, but it's integral to the quality of the experience.
The massages I've gotten there suck, but frankly that's not the main attraction for me. The spa itself is well appointed, with hot, warm, and cold jacuzzis, huge steamroom, and a large dry sauna.
I was there in November 2007 and they had installed white floodlights in the dry sauna. Very annoying -- if you're lying down in there and trying to relax, you feel like you're sitting on the floor of Staples Center. They used to have dim lighting which made it more conducive to relaxation. I don't know whose boneheaded decision that was. Could make me go in search of a different spa.