TBH you can either go semi-tank or glass canon with Kass, it just depends on you're play style.

When you play semi-tank you do a bit of sustained damage and when people run away you seal the deal through a combination of blink, silence, and slow w/ light mana drain.

When you play Kassadin, you jump into team fights, take out one squishy and run. Often time you can divide the enemy team too, getting some to follow you (if you're not in a high tier game that is).

Both styles are viable, although the glass canon makes farming (champions) easier, and the tank-ish build is a bit easier to play. I personally run a 1/2 and 1/2 build with an early RoA followed by a DFG/Void Staff/Abyssal Scepter. Almost universally, Kass builds take Catalyst early anyways, so why not build it into something useful?

The second RoA seems a bit like overkill to me though... if you buy it at like minute 30, it will never build up to its full potential (and therefore it is an inefficient item). For the same cost, you could buy Rylai's, which is like a maxed out RoA w/o mana that adds a slow to void sphere and riftwalk (which is actually pretty good for chasing, ganking, etc).

clarity is good for stacking riftwalks.

Oh and dont forget that riftwalk is wasting more and more mp the more you use it before the stacks end. You can add stacks when traveling and do heavy damage if you time it right, but at the cost of a considerable amount of mana.

Anyways I start with dorans ring then roa with mejias. That's all he really needs. Next is some magic pen, which one depends on how much magic resist is being used for me, and then either zhonyas or lich blade. You have to be moving blinking around when your null sphere is on cd and in team fights dont forget to constantly throw out force pulse.

Guardian Angel is really really good on Kass I almost always get it as my 6th item ^_^

another tip to know is that you have to know who to throw your null sphere to, its usually the other mage, but sometimes its awesome on shaco and tryn.

Kassadin actually requires a lot skill to use, hence the inability for many pepole to play him. First of all, he is an assassin, if there is nothing to assassin you are screwed, you need your team. (nothing new here, a tank needs his team to dps, a dpser needs his team to tank and you need your team to bring enough distraction/damage so you can kill potential dangerous heroes off).

Current mejai is epic on kassadin, your rift walk keeps you alive from many situation and you ap ratio's are epic (0.7).

Cookie cutter build:

Catalyst
boots
Mejai
Upgraded boots (sorcerers)
Rod Of Ages
Void staff/zhonya (all those kind of ap items)

Fill mejai, own.

Few hints: Don't attempt to be cool before six, you are gimped till then. You have 2 spells for damage, forget about rift walk damage the mana / damage is ******ed. Use it as damage when oppurtinity rise but don't expect it to be damage. Nice tricks, building up rifts in health fountain, it rarely works. It's as cool as ashe arrow + teleport but nothing useful and usually risk/reward is not worth it.

Gain AP, that's the trick really. Ps. Sheen/lichbane look like a cool optionm, but reality teaches that you have no the time or tankability to get that close to heroes, you can be satisfied if you drop your force pulse + null sphere and get out after.

If you have troubles getting those first kills: AP/level is actually good on this hero. AP/lvl on yellow/blue/quents + magic penetration on red makes you a danger from start on. Some mejai stacks and you win a game.