Yep. Quick question

Yes. Ever since I've gone back to playing kass mid, I've been going double dorans ring then I rush archangel's. Your mana pool keeps growing and you gain significant AP, not to forget kassadin's damage is off the charts haha. Then I go for deathcap, and the game is often over before I finish it. I used to get archangels after both RoA and Rabadon's, but I'm VERY glad I changed it up.

Yeah, but it's not really worth it because you can't spam abilities much to charge up the Tear.

Generally most Kassadins would go Catalyst > Rod of Ages

My kass build before DFG nerf was something like this

Bootspots > DFG > RoA > Sorc boots > Deathcap > Zhonyas/GA > Situational (Usually Lichbane).

I never really liked Tear on Kassadin because I feel he's such an offensive champion, if he doesn't do enough damage to push his enemy out of lane, then he isn't really doing his job correctly. Why not just zone out the enemy mid and use your Ws passive to regen mana? Also, your jungler should really be giving you blue :P
Late game his job is essentially to blow something quickly, so I don't know why you would need a high mana pool to do that. One combo of DFG > R > Q > E > W and the enemy something should be either DEAD, or unavailable to continue in the fight.

Now these days, I dont feel like rushing DFG is as good as it was so I might build something along these lines:

Bootspots > Kage Pick + Sorcerer boots > RoA > Deathcap > DFG > Zhonyas/GA etc etc.

TDLR : Tear prevents early game nuking and zoning, lategame you don't need the mana.

i build
boots 3 pots>catalyst>Sorc Boots>RoA>Hextech Revolver>WoA>Deathcap>Zhyonyas>Situational
ALWAYS BUY WARDS BTW

DFG > WotA most of the time imo. In lane 6+ the trades should heavily favour you (taking little - 0 response dmg) and late game your dps isnt sustained enough to get any worth out of Wota. Just blow something up from 100%-0 then gtfo.

I love kassadin mid, and lately I've adhered to this combo:
Boots/Pots > Catalyst > Blasting Wand > RoA > Sorc Boots > Tear > Rabbadon's > Ryalai's > Situational items (void staff, banshee's, abyssal scepter, etc).

Getting tear after RoA/boots are completed gives you a very good mana pool midgame, allowing for earlier rifstacks, and if the game continues into the late game, you'll have a very large manapool to reach those max riftstacks.

I just go 4 Archangels and a Deathcap.

why would you get any +mana or mana regen items on kass... do u know what is W does? if u run low... auto attack u just used a mana pot for free getting archanges or tear is just a waste of gold RoA is a good thing to get if u really need the mana so u can survive teamfights but for the most part you should be able to auto attack and get all the mana you need, and i dont know about the people above me but by the time im gettin max riftstacks ive got my W maxed and am hitting constantly inbetween