Karma, do something bad, she'll get you right back... harder. Used to be broken before patch, still broken after patch. Decent cc, aoe, decent in team fights, can even jungle! I play her mid, and that's all i'm gonna go over today ;)
1.) This is the build I use, the one that suits me.
2.) It might not fit you, but I just wanted to share this.
3.) THIS IS A SIMPLE BUILD, NOT DETAILED, NOT HARDCORE. JUST SIMPLE.

ONCE AGAIN, SIMPLE BUILD.

Marks- Greater marks of magic pen (x9)
Seals- Mana regen -or- armor (x9)
Glyphs- Scaling ap (x9)
and obviously, Ap quints (preferably flat)

Why do people even write long guides? i play for fun.
Step 1: Max Q
- 1 point into E
Step 2: Max W
Step 3: Level up E, but try to max out ult.

Q.
Auto.
Q.
Auto.
Q.
Auto
Q.
Auto.
Q.
Auto.
Q.
Auto
Q.
Auto.
Q.
Auto.
Q.
Auto

Standard, (going mid)
Boots x3 pots.
Jungler on enemy? No prob.
2 wards, and all pots.
Bruiser mid?
Cloth, 5 pots.
First B,
If you can, blasting wand and mana crystal.


Core
Rabadons, Rod of Ages, Athene's.

Situational:
Morellonomicon, Void, DFG, Zhonya's. etc.

Try to get:
Rylai's, Will of Ancient.

NEVER
-Archangel's
"BUT IT WURKKKSSS"
Yeah, get it if you want. This is just a skeleton build, but I wouldn't get it.
"BUT KARMA USES ****TONS OF MANA"
And that's why she does such damage. I would spend it on better items, but go ahead.

Standard:
USUALLY IONIAN,but you know what to do with boots.
Rabadon
Rod of Ages
Athene's
Will of the Ancient
Rylai's
And end off with a heavy ap or magic pen.

PROS:
Heal.
CC.
Kite.
AOE damage/shield.

CONS:
slow start, but damages.
Heavy mana expenditure.
Somewhat reliance on ult.