For a quick coffee while touring the Principality, there's nowhere better, and you can sit in the square watching the tourists dribble over the vast array of supercars parked outside the Casino and Hotel de Paris. The hot chocolate is especially good, served in a jug rather than just a small cup.
As for dinner - that was more of a mixed blessing really.
Firstly, it's a good idea to dress for dinner - while there seemed to be no public statement of the fact, simply everyone, French or otherwise, were outfitted in suits and dresses.
Despite its size, service from the waistcoated staff is quick and attentive.
Our starters were both excellent - Gerry went for their onion soup, which was fresh and tasty (though not as deeply flavoured as others) and I chose a soupe de poisson, served with side dishes of grated cheese, croutons and rouille. It was fully flavoured, and deeply satisfying. First class. However, while my main course of grilled whole seabass was as good as my starter, Gerry's chicken was disappointing - slightly overcooked and a little dry.
The meal was washed down with a bottle of white, and we skipped desserts.
Overall, this meal was an enjoyable experience, but not really worth the 135-Euro price tag. It was merely a good meal, and for that sort of cost in the UK I would expect excellence, which sadly wasn't delivered.
In conclusion - you should definitely visit Cafe de Paris. But go there during the day for coffee, and find somewhere else for dinner.