The sticking point about guns in a fantasy setting is, for me, at what tech level do you introduce them? And what keeps them there? There has to be some kind of underlying internal logic.
First, some bright spark figures out that if you stick some alchemical reagents into a tube and shove some rocks down there, then set it on fire, and you get a big kaboom. Congratulations, the earliest firearms are born. They are expensive, unreliable, inaccurate, and slow.
Is this the right place to keep them?
Guns like that aren't much fun, so most people will want to move things along by a couple of hundred years to smoothbore flintlocks. Now the guns are handmade by skilled craftsmen, the powder is reasonably common, and accuracy is greatly improved by sights, but the guns are still slow, taking perhaps 30 seconds to reload after each shot. Anyone can learn to use one with a couple of days training, so finding someone who bothers to practice with a sling or a longbow should be really rare. At this point we begin hearing cries of protest from the magic-loving folks: "but I WANT my longbow. Its faster, and my character spent a lot of years learning it. Why? umm...because its faster?" "doesn't that mean there is starting to be industry polluting the wilds and making the druids and elves angry?" Maybe it does. On the other side are the players who really insist they can still use their high base attack bonus and attack 3 times in a round with a pistol in either hand, even though I would personally love to see someone post a Youtube video of someone simultaneously loading 2 flintlock pistols in the middle of a melee. At any rate, this seems to be the point that Paizo is thinking about with the Gunslinger, and it does cause complications, but not nearly as many as when someone insists on setting up the magical assembly line and advancing to....
The repeating firearm. Break out the ten gallon hats and gaslights, because now we have a weapon that trumps swords, bows, and most magic, can only be produced with the help of an extensive industrial base that doesn't fit into most published settings, and suggests something like Randall Garret's "Lord Darcy" novels, or the Deadlands game.
So the question for me is, where do you stop the progression, and why there? There has to be a reason not to let them progress along to become the driving force in the setting. Arcane magic spells in Pathfinder are cast by an elite few, who carefully guard their secrets, or who have a gift carried in the blood. Divine spells are given directly by the gods to their chosen champions. Skill in muscle powered weapons comes from years of dedicated training. Guns are the product of human innovation, and are the Great Equalizer. They rendered the warrior who trained for combat his whole life obsolete, because by picking up a gun, a peasant could kill him by squeezing one finger (remember Indiana Jones facing the big guy with the impressive sword skills in "Raiders of the Lost Ark"? bang. No more swordsman). In a game that is supposed to be about being a special hero, someone who can change the course of nations by facing deadly foes, that aspect of the game becomes harder to convey the more guns come into play, and the more sophisticated they are.
I don't have a problem with adding guns, but I do think not enough people are thinking about the door they are opening in a fantasy setting. You may want to gun down the ogres with your new hand-cannon, but what stops somebody from selling the ogres some guns? BIG guns. For that matter, there should be no complaints when the dragons start strapping on cannons to use during their strafing runs, or dropping bombs from high altitude. Kobolds will happily pull out some grenades to get touch attacks and AoE damage on those mean ol' adventurers.
If you think the players complained about a spiked pit trap they didn't expect, how are they going to like it when you tell them their torches just set off those barrels of gunpowder the goblins threw down on them?