Deus Ex's Mini Crossbow is availiable early in the game; it takes up one inventory slot, and is useful in some situations.
It's main benefits are:
Firstly, it only takes one hit, if the target is vulnerable. Most not-augnented humans are vulnerable, and if you hit them with the tranquilizer darts, you can run around a corner or two, and they'll be knocked out before they reach you. This also removes the problem of recoil inaccuracy; you only have to hit them once, so recoil ruining your aim doesn't matter.
Secondly, it is silent. A lot of weapons (at least until you add the silencer mod) are noisy, and get alarms set off, etc. The mini crossbow is silent, from the start.
Finally, it renders enemes unconsious, not dead. This is important when dealing with MiBs, because if you kill them, they explode. If you're in a situation where an explosion could be damaging, consider firing off a few darts and running like hell.
The disadvantages are:
Kills are slow. After you hit your enemy, they have time to turn around and hit you, especially in enclosed areas.
Range. The range starts short. You can improve it with range mods, but it isn't up there with the sniper rifle.
Invulnerability. As you progress in the game, you'll come across augmented enemies. They aren't vulnerable, but they will attack you if you hit them.
Overall, I find it a weapon worth keeping hold of. It only takes one inventory slot, and can be upgraded to be a handy weapon. Don't be afraid to drop it later, though, when inventory space is valuable.