We didn't stay at the Cosmo but we did pass through to check it out.
I have to say... the decor is a bit much. I realize they're trying to be hoity toity but it's quite gaudy. Like, hanging-sheets-of-jewels-from-the-ceiling gaudy. It rather reminded me of something from an old 70's gambling movie.
As we trolled around, we didn't really see much that would make us want to pick the Cosmopolitan as one of our hotel choices, but they do seem to offer plenty of amenities (pools, easy access to the Strip, and large hotel rooms) and they even offer views of the Bellagio fountains.
So, I suppose if this style is your thing, then go for it. But for us... we'll stay elsewhere.