The spa will get it's own review, and for good reason, they do a good job of taking care of the customer, especially on the ladies side. The rooms are very modern but well appointed. Good - The hotel is beautiful, there isn't one view toward the city that is bad. I would recommend this resort to travelers that only want to spend a minimal part of their trip in the city and avoid the crowds of Vegas, but if you're getting in a full price I would take a moment to consider other options as well.
Overall the resort is exactly what you would expect from a Las Vegas resort, new, trendy, clean, and well staffed. Our stay was over 3 nights from Thursday - Sunday.
My wife and I stayed at the M two weekends ago for our first anniversary.