Oh yeah... Partisans! I think they add such a cool element to the game! Check this out - I'm going hardcore for a science victory on emperor - trying to get the earliest time I can. Really min/maxing the hammers and gold that I'm putting in - almost no military or gold production, really heavy on science and hammer production. But I got my 8 or so crossbows on the border, along with a few swordsman... sure I should have upgraded both of them, but my military strength is still 150% of the second strongest guy, and rather spend the gold on the buildings in the districts so I can use the hammers on some wonders that will get things swinging HARD! Modern era and looking at breaking a turn 160 science victory... Then boom! I got 4 partisan infantry running amuck across my lands and have nothing to do about 'em! A game that seemed pretty much decided at turn 75 gets interesting again.
Point is, it's a good balancing mechanic of the game. When you're playing below your level to see how far your min/maxing can take you, there are still some surprises that can come up and turn you mentality of "I've been auto-piloting for the past hour, everything is in place" into "holy bejeezus... whadda I do about this?"
A few questions about the mechanic though. First, what is the spy mission that generates these? I'd love to play on deity as Catherine and mess with Korea in this fashion. Second, when you succeed in spawning partisans, what determines the units that spawn? Is it the current "global era unit", or is it the strongest unit that the target civ has access to? And does class of unit matter? Resource dependency? don't know the specifics of this game mechanic and would like to learn.