Really it pretty much depends on how you handle it.
Mostly I am for it,
I ask my players how they felt about it and they mostly didn't care or have much opinion about it. I have so far decided to waite on having it make an appearance.
Freeport has rules for it and so does the Iron kingdoms, so it Can fit into a fantasy setting. It could also appear in the hands of the enemy, such as Sarumans use of it.
If it starts to ruin the feel of the game for the players have them set out to stop the ones using it. If they can stop it before it becomes widespread then they will be able to contribute something to the development of the campaign.
Since magic is most likely more prevelant in a game without gunpowder if it pops up as a unique resource for a group or individual the impact initially will be small. Most people will just think it is a nother kind of magic. "boomstick magick!"
Dwarves are always a good candidate for the 'good guys' having it and they guard the secret carefully, so carefully that the few dwarves that know the secret could easily all be killed and the secret lost. This could be another source of adventure for the pcs. Maybe someone has been murdering the dwarves and they have to figure out who and stop them.
All in all it's up to you, just decide on how it impacts the game and get a feel for it. You can always try to adjust if you don't like the way it goes.
There was en episode of Hercules: TLJ where there was a guy that knew how to make it. I believe he called it "helios"(?) The secret died with him.
Also you can see the effects in Robin Hood Prince of Thieves and how it affected a medival group. Although, of course there was no "big booming magic" in that setting.
SO really try to get a feel for what it will do to the atmosphere of your game. I have set a precedent IMC by introducing a dwarf that is a clever inventor. He has developed a barrell grenade (that so far the pcs have been afraid to use) and a spring loaded "crossbow" (short range, high impact bolt-thrower).
But at his point I don't think I will be fully intigrating guns and gunpowder. I will however use it when I advance the setting and run the Witchfire Trilogy and the Freeport Trilogy.
That'll be a while though.
Well anyhow, that's enough rambling!
Good luck!