I have checked out a wide variety of material related to using fire arms in a campaign, everything from the stuff in the DMG to Iron Kingdoms to Sorcery and Steam. Has any one used blackpowder weapons in a game? If so what rule set do you use? Why?
I've used, and plan to continue using, the rules in Green Ronin's Freeport book. These were also printed in the Dragon Magazine Annual for 2001? 2002? -- the d20 one, anyway, with the Wheel of Time cover art.
I like them because there is a carrot to use them -- they do good damage, but there is a several round reload penalty, they have the potential to explode, and they have a fair amount of variety. They add lots of flavor, are useful, but not too useful to the point that they are an automatic choice over non-blackpowder ranged weapons. I also have them (slightly modified, but really just to eliminate the Freeport specific terminology) on my OGL-compliant website for a campaign setting that has since morphed into something else right here so you can read them and judge for yourself.
I still have some misgivens about guns in my games, both are good and I have been going with the Freeport rules myself for the reason stated by Joshua Dyal, I may move to the Iron Kingdom rules when the setting comes out.