You should expect the Mistral when the pressure over the Mediterranean is lower than the pressure North of the Alps. The Rhone valley is the only real gap in the mountains, so a strong North wind develops. As bluskis says, it can be very powerful, especially in the narrower parts of the valley. It is invariably accompanied by blue skies and good visibility.
When the wind is at its strongest, you want to be on the ground somewhere near the hangar your aeroplane is in. In milder conditions, flying is quite feasible and fun if you're going downwind! It's a drag if you're trying to head North, or if you are crossing the Rhone valley (I did it recently in a small Jodel, and had 40 degrees of drift....)
The Rhone valley also has very strong Southerly winds sometimes, which often bring bad weather.
By the way, you're probably aware that you need to file a flight plan for all flights to Cannes regardless of the origin of the flight. This is part of the (barmy) French anti-terrorism measures. The same rules apply to a number of other airports in France, including Le Bourget, Lyon Bron, Rouen.... Check the Notams!