I stayed at the Monte Carlo a couple of years ago for my bachelor party. All involved seemed to like it just fine. As others have said, it's nothing extraordinary. I don't remember the rooms being especially small. I thought that they were average.

The casino is quite nice in that it's laid out simply (i.e., it's hard to get lost, unlike the MGM).

The location of the Monte Carlo is very good because you can walk with relative ease all the way to Mandalay Bay at one end of the Strip and, with the help of the tram to Bellagio, walk all the way to the other end of the Strip.

If you found a good rate there, I'd definitely advise you to take it.