first items:
if you have to play more passively and want to focus on the mid-lategame
if you think that you can play more aggressively and want to focus on denying cs poking them and forcing them under their tower
second items:
if you built tear first
if you built ludens first and want to snowball harder ( your stuck with 20% - 35% cdr for a long time though )
if you need more vision, pick-potential or a safe way to facecheck bushes and you can take the hit in the damage department
if the enemy has really good engage tools and you need additional help to escort your teammates out of danger
(but imo
>
in 90% of all cases. You are already pretty fast)
if you built tear first and want to scale even harder and slower. It´s a great Item on paper but I never get to build it. Extra HP and Mana is nice, but it´s too slow and offers no cdr.
Bans:
good matchups (adc):
immobile scaling adcs you can hit reliably with your E - double bomb - combo you can force flashes pretty early and with no escape they have to give up cs or risk getting ganked

good matchups (support):
immobile burst mages and engage tanks with huge hitboxes. Many people will tell you that zil is weak to hard engages and that is true in lane, where they can force a fight at any time and can ask for jgl assistance, but especially in later stages of the game they will often tunnel vision into an engage and bait their entire team into piling on top of each other, which is so easy to punish if you can hit those double bombs.
with enough experience even hook supports
First you have to make sure that the only one who gets engaged on is your adc and not you. then it depends on your reaction time. If you can manage to focus the enemy adc quick enough, so that he doesn´t get a lot of free dmg on to your adc, you can often salvage it and turn it into a kill for you. After all blitz doesn´t deal much dmg on his own.
bad matchups (adc)
mobile lane bullies or mobile champs in general
even
times. when her tumble is on cd it becomes easy, but it can become tricky when she uses her ult and there are some really good vayne players out there.
bad matchups (support)
nearly every enchanter. They outpoke you early and can negate your poke with their sustain and shields. Luckily most of them are immobile as well, so you have to farm it out and wait for your jgl.

honorable mentions to spellshield users like
and 
they are not all that threatening, but not being able to stun or having to bait those shields out can be pretty annoying.
also jglers you want to see on the enemy´s team:

some random things to round it all up:
most people max Q-->E--->W but found more success with Q-->W--->E I played both playstyles over a long period of time and the difference was like day and night when I went back to maxing W over E again. I am not saying that you should do the same, but maybe you should at least try it out and see if it suits your playstyle more.
Q-->E--->W: more reliable as it makes picking up targets easier and increases your gank potential. your jgler will thank you. (team oriented)
Q-->W--->E: more bombs and weak slows. better for long and drawn out fights as you can use your stun more often. (more selfish)
And if you choose to play aggressive don´t forget to ward the shit out of the enemy jgl.
hope it helps
[slayer-pantheon-thumbs]