Well , I find him a good champion , with an unique play style. Despite the fact that his buff/nerf history tends to say he was gutted , he actually has significant buffs to his core items , and entirely new options of build paths.
Rod of Ages offers more AP and mana at the exchange of a few HPs. Archangel Staff got buffed by a noticeable amount of AP , making his tear build pretty good. Luden's Echo traded AP for more mobility , helping Kassadin to roam more easily without his ult.
AP item's rework gave Kassadin a lot more toys to play with his enemies.
As a side note , has anyone tried going triple Doran's Ring on him , instead of crystalline flask with components of catalyst? Just try it , and you will find that high scaling abilities aren't good only in the late game ;)
All I can say is that as a Kassadin , you should adapt to every situation. You have the choices between scaling into a late game monster ( Rod of Ages rush + Tear of the Goddess ) or a mid game Assassin that has the potential to snowball the whole game ( Doran's Rings + Luden's )
I personally found it great to have two Doran's Rings , then you can easily sneak a Tear if you get ahead , or just build Rod of Ages, and then what you need.
Honorable mentions:
Ionian Boots of Lucidity: sometimes having your combo ready more often can help you more than higher damage in one rotation.
Abyssal Scepter: do you have a hard time vs magical damage dealers . Maybe you need some more penetration(damage)? This item can help you.
Haunting Guise: good for extra penetration and HP. Also , its upgrade can be good vs bruisers .
Rylai's Crystal Scepter: good for chasing targets, gives a big amount of AP and Health , and can be good at helping your teammates kite.
And many more items that every mage buys , or have synergy with his kit(Sheen items for example).
