Even without range, it's a rare AP mid that can zone him if played well (hermdinger is one of the only ones I regularly lose lane hard against). He can trade very well with his passive. Provided both of you ran Magic pen and MR runes he should win out with his passive taking 15% less magic damage. He can take auto attack harass sure, but I usually run armor runes with kassadin so it's not as painful. Plus if you get a few W auto attacks off in return most opponents will lose that trade. If they don't run magic pen (or for some stupid reason MR) then they are in for a tough time even before 6.
Watch Ocelote play Kassadin sometime. It's a good show (he's doing it right now. http://www.own3d.tv/SKGaming/live/2721 )
At work so no streams for me :|
I don't mean to say that Kassadin is bad or anything. He's like one of my favorite mids. Just saying that he can be counter-picked rather easily so that's something to keep in mind. If you outplay your opponent, or just in an even or better matchup, he'll be a monster. He's frightening to the point that he gets counter picked if he's up.
edit: good catch on the Stacking. I realized that after I posted. Forgot stacks don't scale. It's still a buff the way I play Kassadin. You're in for a lot of basing if you stack that much.
Yeah. Generally speaking, it should totally be a buff as your AP climbs.
double edit: What level are you up to? It would be fun to play sometime. For as new as you are, you're really picking up on the bigger picture well. I'm only a little more experienced tbh. Only started playing in Dec.
Low 20s still! Hooray! I haven't dedicated enough time to just grinding EXP so I can get to the horror that is ranked play that everyone keeps saying it is :|