Kassadin is a very good AP mid laner that I consider to be an assasin if played right. Not many people play Kassadin as far as I know, but i believe that Kassadin should be at least in the top 10 best mid laners at the moment.

Rod of Ages and Morellonomicon are must items for Kassadin because Rod of Ages gives him health, ap, and more mana to use more riftwalks. Morellonomicon because it gives more mana, CDR, and good ap. Also Abyssal Scepter is also a good item if the enemy teaqm has some ap.

A good way to use your Thunder Lords is to AA Q AA and that will help with trades, mostly if the enemy laner isn't ranged. Laning against a ranged champion will be a little harder, but try to play attention to your own minions to see when they over step for cs. Once you have level 6, you will have a great power spike, mostly if you have Rod of Ages. Once level 6, try to poke the enemy with lots of Qs and if possible some Ws. Once they are about half or 3/4 HP, then you can riftwalk towards them, use your w, AA, Q, AA then if they aren't dead then riftwalk a second time.

Pros:

Great burst damage
Has an escape if getting ganked
Has a shield for lane trades
Able to dodge high cc with riftwalk
Has a gap closer

Cons:

Hard to lane against ranged champions
Vulnerable to ganks early game since he doen't have his riftwalk till level 6