It's a viable sidegrade.
It makes sniping more reliable (no need to headshot) and gives you a more supportive role.
The jarate effect is especially useful by making enemies retreat and scaring them away.
Extra charge speed is there for help the sniper do some basic picking, 150 bodyshot can easily take out medics and scouts.
You can extinguish teammates by shooting them.
Headshotting and the explosion of jarate is nice for pushes (like enemies on the cart or going through a choke), it will coat all of the enemies.
It's great against DR spies, since marks them when they feign death. Additionally, you can apply jarate to ubered enemies!
They won't take damage during the uber obviously, but the jarate will probably finish them off once the uber fades (for best results, apply near end of uber). The pocket probably won't even notice that his buddy is covered in jarate. Do note that applying jarate to the pocket won't last very long, due to the healing.
Finally, if you're worried about enemy snipers (because their stock can kill you in a quickscope headshot, use the danger shield.
Also, many of the guides on the internet are outdated, it has since been updated to apply jarate on unscoped shots and explosions on headshots. (thanks to Robotnik for reminding me of this)
Taken from an answer I wrote on reddit a long time ago, link here.