I LOVED our room but did not care for the casino.
Our room faced the Bellagio fountains where we had a great view of the fountain show every night and the room itself was a mini suite decorated well with a small kitchenette huge bathrooms and two large flat screen TVs. Our room even had a large bath tub facing out the window so you could watch the Bellagio Fountains while enjoying a bath. I would say it was by far one of the nicest rooms I've stayed at while in Vegas (we've stayed at Mandalay and Aria). The balcony was the most amazing feature of our room. We had a perfect view of the Vegas strip!
So some things that I did not care for:
The Casino was small. On a busy night all the tables were filled, i had to go to Aria or Bellagio to gamble. There's no poker room here either. The registration/valet desk was on the complete opposite side of the property so we had to walk across the whole Casino every time we went anywhere. Would I stay at the Cosmo again, yes probably because the room was awesome but I think I would rather stay at Aria because of its larger casino and its central to a lot more things.