I'm pissed to see some comments about the fact that concentration is useless. Especially that it comes from hippies (who was) leveling in inner and who hasn't tested properly how the things work in long term and in warzone.
So i feel like i need to put you in the skin of a 88 conc markswoman in order to make you understand the opposite (im not here to brag about the RP i made, just check the crit hit rate), and no i don't use point shot here, a small group vs group fight near PN2 :
Notice on this one how the hit rate drops once eagle eye is down :
As you noticed, i can even pay myself the luxury to have DS to 3.
The thing is that you'll only see the benefits of conc if you use normal hits - if you do normal hit/skill/normal hit/skill then concentration won't do what you're waiting for, excepted increasing your skill and hit chance rate.
I want to point this out not only because what is said is wrong, but because
people saying this are dangerous for the overall balance of this game since the NGD's feedback about balance comes from the players.
Have fun