Janissaries didn't exactly fit our definition of slavery. They were forcefully conscripted Christians that made up an elite and disciplined corps. They were the sultan's honor guard, after all. To say they were slaves is a stretch. In English they were "door servants," which you can say is neither freeman nor slave. They underwent strict training but were paid regular salaries with bonuses.

More or less they took pride in their ferocity in combat and strict military discipline and tradition. It wasn't until the gradual 'civilianization' (essentially the watering down) of the janissary corps that there was true dissension in the ranks. By the early 19th century the Ottoman empire had been trying to modernize with the janissaries being a huge point of contention. Almost all janissaries were so stuck in tradition they refused modern uniforms, weapons and used dated tactics. They were outdated.

By 1826 janissaries had been reduced from upper-class members of society and over 135,000 revolted against Sultan Mahmud II. It was suppressed with 6,000 dead and afterwards the entire corps was disbanded in favor of a more modern army.

Edit: Apologies for no source at the moment. Currently travelling. Will add proper links for the numbers.

As HolyWarDance points out, they were not slaves as the West saw slaves, but yes, technically they were slaves. With a difference; their status was much higher than an ordinary domestic slave.

Lord Kinross wrote The Ottoman Centuries, which is a good general outline of the Ottoman Empire.

The Janissary men were recruited by force. But they got a lifelong career, social prestige, and an education. Any of those was very hard to come by among the Christian Balkan populations. And they were recruited in a confused theater of war. Kinross points out that Muslim armies often had Christian contingents, Muslims might fight for a Christian ruler, and the system of soldier-slavery was not brutal by the standards of the age.

They got a standard of living that was higher than most people. They ate well, and had "espirit de corp." Their symbols were the camp kettle and spoon, to demonstrate their standard of living. They could not marry while in active service, but could when they retired, and when old, got pensions.

In [Edit: individual] poverty, as a group they were well off in lodgings and clothing. They became politically powerful by the time of Mehmed II. It was customary for new Sultans to give them a raise or a bonus. When they rebelled, the way they signified it was by turning their camp kettles upside down.