Played this three times recently. As the French player, I attacked the militia on the Spanish left, routing it off the map. This was followed by an attack on the Spanish right (dictated by the cards) which routed several units into the River Trueba for a 7-4 win.
Not quite sure how it happened, but the return game saw a rapid 7-1 Spanish win. A French assault towards the river saw it beaten back and the militia held its own on the left. In my experience of this and C&C Tricorne, most players hold militia units back to protect them, but if properly supported, they can be very effective.
So we played again, my opponent confident he could do better. He did, but still lost 7-3! Once again, a French assault on the Spanish right in front of the river was beaten back with heavy loss. The French cavalry came forward and destroyed a line unit in square, but when subsequently attacked, the militia held it own without forming square, and the coup de grace came with two Spanish cavalry charges eliminating both French light cavalry units.
Much has been written about the guerrilla rule. In each of the above games, the Spanish drew an extra token, for a total of two. Did they swing the games? Probably not. In both games as the Spanish, I used the guerrillas to stop a French attack dead in its tracks. As the French, I drew the guerrilla tokens out by making threatening moves with weaker cards, allowing me subsequently to play stronger cards (bayonet charge and rally) without fear of their being annulled. People get exercised about this being ahistorical in that guerrillas were rarely present on a a battlefield, but hey, it's a play-balance mechanism so who cares!