Kassadin is one of the few champs I feel comfortable taking Mid Lane and having a decent chance of doing well, if not in lane, then at least through roaming to other lanes.

I've been playing him off an on even before his most recent re-work. I've been called on to do mid duties more of late so I've dusted him off to try and terrorize the Rift.

It is hard to described the differences in before an after the most recent rework. His mobility is definitely lower. His ultimate feels clunkier. It is harder to get in position to insta-gib someone and his chase potential while still good, is definitely diminished.

Damage-wise his ultimate feels relatively anemic since it scales off mana and besides Rod of Ages and possibly Lich Bane you don't build a whole lot of mana, since going Tear of the Goddess into Rod of Ages leaves you exceedingly weak in the damage department until at least mid-game. The gating on Riftwalk and Force Pulse also makes it hard to stack Tear efficiently.

Null Sphere is good for poking, last hitting unsafe minions, setting up favorable trades against magic damage dealers via the shield it provides, and interrupting channeled abilities. Hello Katarina my name is Kassadin.

Nether Blade - this thing is a god send. It has better range than your base autoattack and restores a percentage of your missing mana (x5 if you hit a champion with it). It is also an autoattack reset, which can help with last hitting. Use it to last hit a minion every time it is off cool down.

Force Pulse - is gated by the need for 6 spells to be cast in Kassadin's vicinity before it becomes available to use. This is more of an issue in lane than it is in skirmishes or team fights when spells are flying from both sides. It is a potent spell so some conditions on its usage are warranted.

Void Stone as a melee champion and assassin, being able to ignore unit collision is lovely. A built in 15% magic damage reduction is situational, but in 90% of your games, someone is going to be doing at least a little magic damage to you (screw you Garen!).

Team Fights

If someone is out of position, erase them. If you've got well defined lines of battle, then use Null Sphere and Force Pulse to poke. If a priority target gets low you can Riftwalk in to snuff them out and then pop Zhonya's Hourglass if the rest of their team turns on you before you can Riftwalk out.

Keep an eye out for anyone on the enemy team that has a channeled ability and bonk them with Null Sphere if they try to channel it.

Match-ups

Katarina - you are pretty well setup to take on Katarina. However, the rest of your team probably isn't so you have to follow her if she roams. Try to get your jungler to help you keep her roam paths warded.

Ezreal - hard to pin down, but this lane should be no problem as long as you don't eat every Mystic Shot he throws out. Stay behind minions and go in on him after he has used Arcane Shift either offensively or defensively.

Nidalee - can also be hard to pin down, but you also win this lane as long as you can dodge spears and stay healthy enough to avoid being executed by her if she jumps on you in cougar form. Stay behind minions and go in on her after she has used Pounce either offensively or defensively.

Lux - run 21-0-9 masteries to dodge her skill shots. Proceed to murder her once you hit 6th level, push your lane and go terrorize another lane.

Zed - is annoying as he does a lot of damage and has sneaky ways to get at you to do damage. Your job is to keep up with him in levels so that when he gets his ultimate, Death Mark, you can simply Riftwalk away from him. He will probably be able to out push you and roam. If you think this is happening, clear you wave as fast as you can and follow him.

Item Builds

vs AP
Crystalline Flask
3x Health Potions

into Rod of Ages

vs AD
Cloth Armor
4x Health Potions
1x Mana Potions

into Seeker's Armguard then Rod of Ages unless you really need the active from Zhonya's Hourglass against someone like Zed.

Boots upgraded to Sorcerer's Shoes when it seems appropriate. Along with more mobility, they are a significant damage spike in lieu of the ability to afford other items as it makes sure more of the damage you are already doing gets through.

After that if they have more than one dedicated magic damage dealer, I'll pick up an Abyssal Mask.

If they only have a few sources of magic damage I like to pick up Luden's Tempest as you get more movement speed which is always helpful for an assassin and you charge up its passive quickly with Riftwalk.

Round out your build with Rabadon's Deathcap and/or Void Staff.

So a completed build would look like.

Vs AD

Rod of Ages
Zhonya's Hourglass
Sorcerer's Shoes with Enchantment: Homeguard
Luden's Tempest
Rabadon's Deathcap
Void Staff

Build Seeker's Armguard and then a Rod of Ages

Vs AP

Rod of Ages
Zhonya's Hourglass
Abyssal Mask
Sorcerer's Shoes with Enchantment: Homeguard
Luden's Tempest or Rabadon's Deathcap
Void Staff

Luden's vs Rabadon's comes down to what you feel you need more, the raw AP or the movement speed plus the passive splash damage.

In either build you can toss out the Void Staff for mohr damages or utility if for some silly reason they aren't build much magic resistance.