I was able to use musketeers in STW 1 to great effect. 4-5 units in an army with a good general and I was able to rout enemy units right and left. The enemy would regroup, charge and rout again.

I haven't had the same luck in STW 2, the enemy usually closes to melee long before my units can get a second volley.
Samurai guns are much better and their rapid fire ability is pretty powerful, but I havent had many battles with them.
I haven't tried the monk version.
The best advice I've seen is to set up your army with gaps, so the matchlocks will retreat back through the gaps and then will fire through the gaps.
I have a Shimazu campaign going at the moment. I'll have to try some different strategies with the matchlocks.