If you play her well she is also a pretty good counter-pick to many top-lane match ups. Take flash-ignite, use her q to wear down the enemy. Once they hit below 1/2 health wait til you have 2 mantra charges and go in for the kill w/ mantra'd e, q, mantra'd e, ignite. When they come back to lane, keep the pressure on so they can't catch up. Also shielding minions can deny last hits. Build her as a tanky mage (similar to the way one would build morgana). Her early and mid-game burst is amazing and unexpected, but since her role changes to more support-y and sustained damage late game it's wise to only use her on a team that has other significant sources of burst magic damage.

One important note: Karma's support presence is very strong, but it comes off in a way that tends to inspire overconfidence in extended skirmishes. If you start to get nervous, that's a sign it's time to disengage. Your teammates will feel invincible and they will stay in a skirmish as long as you are present, but they don't know how to gauge your resource management, so it's up to you to let your team know it's time to leave. I can't tell you how many times myself and 2 others have got 2-3 kills over the course of a few minutes of skirmishing, only to give up an eventual triple kill.

woah, I have never even seen someone utter the word Karma, let alone buy her XD

I like support Karma, whatever folk say. Sure, she's no Lulu but neither is anyone who isn't Lulu so come on. The biggest struggle you'll find is she's more a mid to late game support. It takes awhile to get all her skills up to a decent level and it's only then she really shines. Having an AOE % heal, a slow/speed buff, and a damaging shield makes her a great utility support. I find her to be the new Janna since they decided that Janna should be useless for awhile. She's a little harder to control than Lulu and doesn't have quite the same clutch skills.

Still, I'd say pick her up if you want some variety in your support line up. She's fine mid in the same way that Orianna is nice having someone with utility and support to their damage that, like Ori, suffers a little because of it. Your team probably won't pick up on your impact quite as easily as other supports but if you keep up with her I don't think you'll regret the decision.