LoL - What should I build?

Well, this is the basic stats I would put for Kassadin, with the addition of maybe an exhaust boost or something along those lines. I feel as though the cooldown reduction plus the mana and experience gain lets you get to full speed just that little bit quicker, which may help you destroy your enemies later on. As for spells, I would have to go with Exhaust and Ignite. I couldn't even begin to fathom how many times the extra damage on ignite has helped me kill a foe, and with exhaust, you can help your whole team catch up to someone if you need a gank on them. Now, I know Kass has a slow spell already and it usually comes up quite often, but it never hurts to sponge some damage either.

If I were to pick a different spell, I would probably swap Exhaust for Ghost, so you can get everywhere just that much quicker.