We loved our stay at Columbus Monte-Carlo. The staff were excellent, exactly what you would expect at this level of hotel.
The room itself was lovely, really bright and we felt for the money we got a lot more than in other cities we'd stayed in the region.
The pool was really nice and we found it very quiet, it is near the heliport so every now and again you do hear these take off.
The location is great, it's not in the centre but Monaco is so small that the hotel is only a 20 minute walk to the Casino Monte-Carlo, for example.
If you have never been to Monaco, I would recommend not getting the train, especially if you have children. We found it was a rush and a push to get on the train, the station fills up very quickly and frequently.