We visited it twice - first with the tourist train when you can't really get off, train stops trete for 5-10 mins or so, you can get some pictures if you lucky enough to sit on the right side (we were not0.
So we visited it second time when leaving French Riviera as it was on our way. Parked the car in the underground parking just by the square, the visit took us less than am hour, so it was not expensive at all. It was a lovely morning and we took a lot of pictures in the square and the beautiful park nearby. Most people come for this, I suppose.
If you want to see the interior of the Casino, but not bet, they have tours for visitors - you must be at least 18 years old; tickets cost about 7 euro, as I remember; we didn't take it as we were in a hurry because it was a long way to home; but we could step inside and see the grand interior of the entrance hall and even took some pictures.
If you come to Monaco, you mudt see the Casini, of course.
For me, the most amazing fing was the mirror in front of it (artwork by a Chinese artist, I guess): the reflections were fabulous and I could stay there forever making pictures from different angles. It's a very organic modern touch to the 19th sentury square and adds value to the views.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.