By way of full disclosure, I haven't used the spa & salon services at Aria - just the gym, which I hit daily. It's got a massive cardio set-up - I'd guess 18 treadmills, another 10 or so cross-trainers, plus about 8 recumbent and stationary bikes - so you never have to wait for a machine. There's a full circuit of weight machines and a good-sized free weights area too.
The gym is one of the primary reasons I stay at the Aria, helping to compensate for its shortcomings in other areas, which I'll address elsewhere.