My experience with a concussion:
After an explosive breach entered a door that, unknown to me, led to the landing of a staircase going down. Totally missed the inch or two high ledge before the staircase, and took two steps in. Next thing I remember a team mate is shaking me awake. Was out for less than a minute, but the fall snapped my nods mount and cracked the radio battery on the back of my helmet I used for a counterweight (hit the opposite wall on the stair case on the way down). Mates said I looked like a wreck hobbling up the hill with blood coming from my nose and mouth (fall bruised my right hip). Got helped the rest of the way up the hill to our strong point where I felt like throwing up for an hour or so and had a massive head ache. Took 8mg Zofran and 1000mg Tylenol then got back into it. Maybe not a critical shot like pinging someone in the head at distance, but was able to put warning shots a meter off of a target at 900 meters an hour or so after.
In short, life sucks after a hit to the head, but you can still do stuff. Drugs were pretty important in that endeavor as well.