Kat is perfectly fine. She has a great early game harass (much like Ashe's volley), and can scale well into late game.
But she has one of the easiest interuptable ults besides Nunu.
It seems like before she was buffed she was cced more during her ult than after the buff. I see too many CC heroes pretty much ignore her and just try to run away instead of hitting her with a stun/silence/knock-up/etc and destroying her. And so the person is like standing in her circle of dagger doom while typing /all OMG KATS OPZ.
Another mistake I see people do is just stand there while she Qs the team over and over and over again. Don't stand near minions, it bounces off of minions depending on the level of it, and most Kat's increase it to lvl 5 asap. So if someone just sits at their tower and lets her Q over and over again while their health is decreasing and she runs in and ults to only shunpo away, of course she's gonna seem op. It similar to Ashe vollying someone to near death, ulting, and running away while they are under their tower. Or Fiddle's Dark Wind has the same generally harass/bounce.
And to counter AP Kat is really simple, her Q doesn't even hurt with full AP. Neither does her auto-attack. Get some MR, CC her during her ult, and chomp on her face or run away.
She's such a simple counter its ridiculous. And a lot of Kat's don't build health so they are incredibly squishy. They only have an escape if there is something to shunpo too, so she's easy to catch if nothing is around.
This is the one and only time I'll say L2P because I'm sick of stupid people complaining about an OK champ. Kat has never been insane like previous OPs (Fiddle, Twisted Fate, Twitch, etc).