This build will give you alot of survivability early game, as you will most likely be meeting a AP carry in mid-lane. I play very aggressive early as Kassadin silences the opponent leaving him defenseless for a few secounds. The moment I hit level 6 I go for a kill, if the game has gone to my favor or if the other team isn't obviously superiour and I don't stand a chance, in which situation I normally call out for a gank. As soon as you have the chalice of harmony you should try to harass as much as you possibly can, as your mana will be replenishing pretty fast because of it's passive.

When the game has come further and you're close/have full build, the opposing team most likely knows you're very powerfull, and will focus you most of the time. Because of this you should try and stay low untill a fight breaks out, the enter with your ultimate and silence and kill the most dangerous opposing champion.

Kassadin is also a very good assassin, as you can use Flash and your ultimate to pop out of nowhere really quick, or you can engage with your ultimate, kill your target and flash right out again.

Also, obviously Riftwalk is good to get away from certain situations with. For instance if your team is taking on Baron, and you get attacked by the opposing team and you feel like leaving your team behind you can Riftwalk over the edge away from it all.

Also, you should try to get the bluebuff as often as possible, as Kassadins ulti uses ALOT of mana after simultaneous uses. That's why Athene's Grail + Blue buff is perfect for Kassadin, you can spam Riftwalk without having to back because of mana shortage.

Also, try stacking up 3 or 4 Riftwalks before engaging an enemy, as you then will do alot more damage to the enemy, increasing your chances of getting a kill.