Let's talk about Kassadin

I've bought him many months ago, but i never played him until now.

I've tried Dark Harvest (I don't if it's called so in english. That tune where you collect souls), It worked pretty well. Maybe thunderlord or stromrider will be good on him. I will see soon,

But how should I play him exactly? (Ironic, but not so much) Below i'm talking about non draft games, so don't piss me with counter champs.

Vs Mages you can: Q to shield up whenever you re about to get hit and poke the enemy W to regen mana and reset AA when you engage E...it's a slow...right? Well you can do dmg with it R to dodge the Q(ueue)/abilities, dmg. engage, disengage or use it as a free flash I'd max 1st Q becouse it's the only way you can dmg the enemy from safe spots, and shield yourself>W for mana>E

Vs AD midlaners (Zed-Yasuo. Just played against a Zed, If not the fact he gone afk i'd be 4/10)

Q to poke the enemy, shield just doesn't work VS AD W W to regen mana and reset AA when you engage E...it's a slow...right? Well you can do dmg with itR to dodge the Q(ueue)/abilities, dmg. engage, dosengage or use it as a free flash R to dodge the Q(ueue)/abilities, dmg. engage, disengage or use it as a free flash What to max here 1st???

Is he playable top? Or only mid? His dmg anyway felt a bit "eeeh", i'd expect higher dmg from a splash art of his.

What do you think is the "best" (i know it depends on matchup, but generally speaking) build?

And most important, what is Kass' role exactly? An assassin? Roaming mage? Troll pick (free flashes)