The Cho-Ko-Nu's collateral damage works well in aggressive defense - that is, attacking stacks of units. With more survivability than a Catapult, and a strength bonus against Melee units, the Cho-Ko-Nu makes a great stack-destroyer. At this stage of the game, it makes the Chinese hard to attack.
Cho-Ko-Nu aren't so effective attacking cities defended by longbows, as they cannot be given City Attack promotions. However, they can still be used as part of a combined-arms force, protecting attacking Melee and Siege units from Macemen and similar threats. With Cho-Ko-Nus against archers, however, the collateral damage keeps increasing the odds, which are usually already high to begin with. Cover promotions remove their biggest counter, but some will need Formation to counter Horse Archers, which are immune to first strikes.
Defensively, the Cho-Ko-Nu loses its advantage. Alone, it's easily picked off by mounted units, and in cities, it's not much more dangerous than a Longbowman. If attacking China at this point in the game, give some Crossbowmen or Pikemen Cover promotions to protect stacks, and give stacks some Medic promotions to counteract any collateral damage.
Repeating crossbows have a long history, with the oldest accurate written record dating to the Han dynasty (ca. 20-220 A.D.) in China. The Chinese repeating crossbow, the Chu-ko-nu, is an extremely simple piece of equipment. It is claimed to have been invented by Chinese strategist Zhuge Liang (181-234 A.D.), which is arguable since the earliest drawings of the weapon have been found from the buried library of Chu, dating all the way back to 250 B.C. The Chinese repeating crossbow saw its last serious action in the Sino-Japanese War of 1894-1895, where photographs show repeating crossbows as common weapons among Manchurian troops. The basic construction of this weapon has remained mostly unchanged since its invention, making it one of the longest-lived mechanical weapons.