I dunno. I just played a streak of Kassadin. I was getting huge amount of kills, to the point of ridiculousness. being able to wipe out starting heroes with silence and slow to prevent them from getting away is pretty handy. Those two spells alone, null w/e and force pulse take out about half my opponents life early game, and if I get a soulstealer, late game I am two shotting them.
Edit: Also, not to mention that his force pulse, while kind of hard to use effectively going one on one, in team fights, being able to spam a slow aoe that does 300-600 damage is extremely good.