Janissaries are solid. I wouldn't call them one of the great UUs, like Praets or Redcoats, but they're definitely in the middle and maybe a bit above.
Ordinary Muskets kind of suck because there are contemporary units that are better at everything. Need a unit to attack cities? Macemen can get those precious City Raider promotions, making them far better than Muskets despite their slightly weaker strength. Need a unit to fend off mounted raiders? The lowly Spearman is nearly as good as a Musket, and much cheaper, while the Pikeman leaves him in the dust. Need to defend a city? The Longbowman performs just as well as the Musket (6+50% vs. 9) and is cheaper.
So, most of us only build Muskets as a backup unit, something you stick in a Stack of Doom as a defender.
But Janissaries... mmm. A Musket with 25% vs. melee, archery and mounted units? That's a Musket that can actually do stuff.
They do require you to beeline Gunpowder, because they're only good as long as nobody's building a lot of gunpowder units. But during that window, they're very nice indeed. Just with Combat I, they're better than City Raider II Macemen. Throw in Cover and they can rip open a city full of Longbowmen like a bag of Cheet-Os.
Also, you no longer have to do that silly scissors-paper-stone thing. You know, "I need Macemen to capture the city, Pikemen to defend against raiders, Crossbowmen to fight off enemy melee units..." No. While the Janissary lasts, you don't really need anything but Janissaries. They're as good as Pikemen at killing cavalry, better than Crossbowmen at killing melee units, and so on. Crank 'em out and stack 'em high.
The only reason they're not in the top five UUs is because they have a short-ish lifespan. But otherwise, they're great.
Waldo