Pros

- Top is more suited to a farming play style so you can get big for late game
- Build better suits Kassadin's the in-your-face kit
- Less early-mid team fighting
- Less AA harassers
- Great split pusher
- You can make better use of your w
- Better survivability than standard Kassadin
- You can easily kite most top laners well with e and ult

Cons

- Will generally deal less damage than the standard Kassadin
- Passive is mostly wasted
- Less roam potential
- longer lane makes early ganks a bit scarier

Flash - Need I explain? Your only means of escape pre-6 and great for the surprise Flash > Riftwalk cross screen blink.

Teleport - For top lane this is self explanatory. For mid I like it 'cause Kassadin's early game isn't very good and this can help if you're getting pushed out of lane and can also get you level 6 sooner in some cases. This is also a good spell for sudden ganks and split pushing.

Exhaust - A good spell to counter burst, which Kassadin don't like too much. Better than Barrier imo because it serves a similar purpose but is more flexible and scales better.