I main mid too, and here is what I think...
Karma mid is circumstantial Imo. I have played her mid several times, and she has ridiculously powerful early-mid game lane bully potential. This is coupled with her powerful MS buff on her shield makes her roam potential extremely strong. If you were to mid karma, you have to bully the opponent out of lane then continue to push your own tower, or assist other lanes with ganks. Like was said earlier, Karma falls off A LOT late game... I mean she doesn't at all compete for damage when it comes to late where people have huge HP and MR to stop her power pokes. The best thing you can do at that point is shift over to a support role. If your team already has a powerful AP mage, Karma can be an excellent pick (vlad Top for example) simply for that fact that they can make up for where karma is lacking which is late-game damage. Karma, however, is extremely good against low-mobility squishy mids like Anivia or Zyra or Syndra. You can easily stomp them with the poke and follow up with a shield + tether combo and mantra inner fire for the KO (you might want to ingite too to guarantee the KO if they aren't quite low enough for an all-in). There's very little they can do at that point, and that's when you start to roam and stomp other lanes with your team...
The point being, you need a team comp that supports Karma and not one that depends on a high-power control-mid like Anivia, Zyra, Veigar, Syndra, etc... The team comp she is better suited for, is one that already had plenty of damage to make up for her lack in that particular area late-game. The good part is that you won't need to bully people out of lane when laning phase has already ended late-game meaning that her role shift is excellent and those high-powered shields can really turn the tide of a team fight.
As a mid player, I would also recommend you specialize in at least 3 different mid picks that have a variety of roles. For example, I play Akali (assassin) as my main, but I also play Anivia (a power control-mid) and Ahri (high mobility tactician/play-maker). I mean it's up to you, but I think it's good to have a nice number of picks behind you so if your champ gets banned out, or you are going up against a team who hard-counters your main, you have another route you can go to better deal with the enemy team comp.
