The impact it makes on my life is the assurance that God really really loves and cares for me - not having had a good father figure it was difficult for me to see God as a father, but Ps. 139 changed my view forever.
For me, a person who did not know God growing up, it is a great comfort to know that He was their through all my trials and tribulations.
Looking back I realize that it was the reason that I survive spiritually and did not get lost forever - because He knew me - He formed me - He was with me all the time - He helped me to go through it.
Even today it is wonderous to think that I am not a mistake or an accident - that I am here because it was God's will. That He wanted me here to make a difference and that He loves me more than I can ever realize.
I am who I am because it is God's will and in that knowledge I can rest and not condemn or judge myself according to the world, or my own expectations.
Psa 139:14 I praise you, for I am fearfully and wonderfully made. Wonderful are your works; my soul knows it very well.