As someone who mains kass, i have to disagree. Early game (lvl 1 - 5) all you have to do is be careful. as soon as you hit level 6, the enemy champ (assuming you took mid) already underestimates you and it makes it that much easier to wombo-combo his rqew and ignite. And in team fights he is way better than brand. Assuming you have good team comp all kassadin has to do is let the tank initiate, and the blow his load all over brand's face. brand being silenced and slowed is an easy target for the rest of your team. Kass can take out enemy mages in 2 secs flat, and his ability to harrass at towers leading to easy kills is incredible.

I build a slightly tankier build on kass Sorcerers boots ROA Rabadons Ryalis Lichbane (defensive item offensive item of choice)

This makes the enemy not want to target him because he can blink, and because he has 2.7 k health, this gives him some survivabilty from CC with a shit ton of ap to go with it.

This might just be me ranting but whatever, to answer your question, i'd say he's a little op. I usually go something like 8/1/12 with kass and just recently i went 24/5/16 AAANNNYYYWWAAAYYSSS don't abandon him, he's a good champ.