The Monte Carlo is average. In any other location it would be a great hotel, but in Vegas it pales in comparison to any of the newer hotels (Bellagio, Mandalay Bay, Paris, Venetian, Luxor, Mirage, Caesars, even Treasure Island)
It's fine to stay in, but it would probably only garner a 'C' in the impress-the-wife test. I'd say the best values would Paris or Venetian.