I've played as France many times in this game, as the Battle of France I find to be immensely fascinating.
There was some argument as to how effective the forts really were and what the benefits were, and I think that's been well articulated in this thread. What I would like to add:
1. The destruction of Fort Eben-Emael was an utter coup, a "natural 20" for the German Army that should, in my opinion, be an event within the game. That was a very unique event that had enormous implications for the Belgian defense (all bad. =P ) But it was an outlier and not the norm. It was well-planned and executed, but not something that was feasible against say, the Maginot line forts.
2. The political dimension cannot be underestimated. Belgium severely weakened the Allied defensive plan by their indecisiveness and stalling. By the time they agreed to the French plan, they were already on the run and their lines were collapsing...which the Germans would exploit fully when they rushed the Ardennes. Had France and Belgium been able to mobilize earlier, the likelyhood that Germany would have enjoyed such a penetration of the allied lines would be greatly diminished, assuming the French would have been able to set up their defense in depth-as-planned.
I've been able to play scenarios with both the French forward deployed and dug in inside Belgium, and also along a reinforced French border. I've had the most luck with the latter scenario, since the Belgians do not fortify their border to the same extent that the French do, and the most natural lines of defense lie further inside France.
One scenario however that did work well...was one in which France did *not* join the Allies, nor did it attempt to create the little Entente. A neutral France saw Germany delay it's declaration of war, and allowed France to build up and preserve it's strength into late 1942. Once Germany and Italy become bogged down in Russia and Greece respectively, then France can go on the offensive, using it's airforce and Navy to sweep the Med and ensure the blockade of Germany. By late 1943/1944 the French army was able to go on the offensive. France by the mid 40s has a good technological advantage that it can use to great effect provided it lives long enough, and if it can sweep the Med with British help (which doesn't seem too hard to do), then it's able to play a role in the Pacific when allied forces there are sorely needed.
In any of these scenarios though, the trouble with France is it's low manpower, exasperated by the demands placed on it by it's large Navy and colonial requirements. The forts were a way to economize on manpower in a defensive war. Poor coordination with Belgium ensured that for as good as the Maginot line was, the Germans could still race around it due to Belgian stubborness, of which the smaller (and backwards) French Army was helpless to resist.
