I'm ambivalent about gunslingers in particular. I don't see anything inherently broken with the gunslinger as a whole - I raise an eyebrow at the pistelero archetype, though. I don't permit gunslingers in my games at the moment, however. While I think guns are a good fit in fantasy, I also prefer good ol' fashioned swords, hammers and fireballs.
What I do permit is the spellslinger. There's a very... scientific feel to guns, in my opinion, and I might be stereotyping slightly but I can imagine a "modern-esque" wizard creating the guns in Pathfinder. I've allowed them in my games, and I'm glad I have. Go go Professor Cain and your Kobold Riflemen Brigade.
Good times.