I generally have a few crossbows in each of my stacks and maybe on border cities until the big gunpowder upgrade, like others have said they do a great job beating up on macemen (and axemen/swordsmen) at low cost. I like to have one or two little pillage sets of a crossbowman and elephant or spear/pikeman (can add a chariot/horseman for quicer pillaging). The computer will inevitably throw way more than their value at them to kill them, which nicely soaks up those annoying AI units that otherwise end up pillaging or snagging a worker.
I generally like longbows better for city defense since they're cheaper to build and get the free +25%, but if I can't trade for feudalism I'll gladly use crossbows. A longbow with city defense, crossbow with first strike, and spear/pikeman with medic makes a nice garrison for a border city, the crossbow or pikeman can catch any stray pillagers, and the different unit types make an attacker hurt some.
Personally, I find the Cho-Ko-Nus only a so-so unit; the collateral damage is nice, but the strength 6 and no city raider promotions makes it hard to roll through cities the way cats and city raider macemen do, so I end up having both for offensive work. They rock against a stacks in the open, a big stack of them will simply annihilate whatever they come up against. Plus once they go obsolete any that don't get upgrades can suicide much more effectively than any other unit. Not bad by any stretch, but not something you'll use for pretty much all of your fighting the way you do Praetorians or Redcoats.