Even in Medieval, as soon as I could get cannons, I carried three or four in my armies. And that was when they were stationary! Now that arty moves, it's even more fun for me to play with. But maybe that's just the Napoleonic wannabe in me.

As others have said, some of the arty works like a particularly-large bore general-seeking sniper rifle. Basilisks and Serpentines are both good for this, but I've even managed to headshot generals with culverins.

And I also have a thing for ribaults. Medieval machinegun, tally ho!

Right, so, to recap: Arty is scary, snipes generals like nobody's business, and few things are as cool as letting fly a volley at 100 yards or so and watching your missile clear a line of men through three different units. And watching half of an enemy unit's first line go down from massed ribaults is cool too.

Of course, I prefer to situate the camera behind the enemy force my cannons are targeting and watch the flares of firing in the distance. "If only they knew what I knew about what's going to be greeting them in a moment...