Karma is awesome. I've been loving her with the build I've been using.
start: Sapphire Crystal + 2 HP pots
she doesn't really have enough mana to sustain herself for long, a sapphire crystal actually helps a lot. And having two pots is very nice over a Doran's even though you have a heal.
then I build catalyst, boots, upgrade catalyst to Rod of ages, Ionian boots, Deathfire grasp, Rylai's/Rabadon's.
Game usually ends by then, if I need more damage/shield definitely go with Rabadon's. If you have trouble catching people/need more hp, go rylais.
And I tend to get a blue potion for some fights if I have enough gold.