Obviously my actual full opinion of ARIA would depend how pricey it was, and I can't say, since I got an almost-free night through myvegas (just had to pay the $45 resort fee). But as far as the room itself, I was quite satisfied: the bed was soft, the basic room was pretty spacious, I loved the wall-to-wall double curtains with electronic control, and the bathtub/shower combination was seriously off the charts awesome (seriously, the bathtub was probably the best bathtub I've ever used - it was enormous, and I loved how it melded in with the shower to give you tons of space to stretch out without worrying about getting water all over).
The hotel as a whole, though, given the choice (and assuming similar price), I'd prefer staying at Cosmopolitan. They felt like they were going for a similar hip-but-classy feel, but Cosmopolitan pulls that *off* way better - more people at checkin so there was no line, less annoying music in the public areas, and of course, way the heck better bars (both in terms of feeling a lot more laid-back and classier, and especially, in terms of cocktail quality/creativity). Cosmopolitan is also right on the strip, whereas ARIA makes you walk through a bunch of overpriced shops at Crystal to get to/from the strip.
So, I totally enjoyed staying here for almost-free, but awesome bathroom or no awesome bathroom, I wouldn't pay $$$ to stay here. I'd totally continue staying here for MyVegas points as long as they'll let me, though (but only if MyVegas doesn't also have Vdara availability when I want to go, as Vdara is totally nicer, in every respect other than the bathtub. Vdara's bath setup is great too, don't get me wrong, and so is Cosmopolitan's, ARIA just does happen to win on that one metric.)