I mean, he's a flippin' ninja.

Zed is a mid lane AD assassin that focuses on securing a strong laning phase after which he can snowball and roam, securing an early to mid game lead by consistently assassinating the enemy carry during teamfights and skirmishes. Zed played with the correct skill and teamwork can end the game early or snowball your team into lategame with a strong advantage.

He has decent waveclear, good laning, strong midgame, and an extremely high skillcap.

Zed is a mechanical champion, whose performance will depend on the mechanical skill of the player and their knowledge of Zed and his matchups, but has an intuitive kit, so he takes a lot of practice to master, but isn't necessarily impossible to pick up.

Currently Zed has fallen out of the competitive Meta, but he is still considerably powerful in Solo Queue

Good times to pick Zed are:
When the enemy mid is a good matchup for Zed AND at least one of the enemy Top/Jungle are squishy
Your team has AP damage to make up for your lack of AP

When you are confident you will win lane

One Important thing to consider:

If you don't think you can win lane as Zed. Don't pick Zed.

Zed needs to win lane, that's the "easy part". The hard part for Zed comes AFTER laning phase. Therefore, if you don't think you can win lane, go with a different pick until you are skilled enough with Zed to take down any opponent in lane phase. If you are in a matchup where Zed loses 1v1 or you are getting camped, start camping bot lane each time your R is up.

Zed is extremely fun and rewarding, and makes all those flashy Faker plays, so you'll be guaranteed to have a great time playing and mastering him!

Pros:
-Easy farming
-Strong lane pressure
-Few counters
-Strong snowball
-One of the highest skillcaps
-Decent waveclear
-Excellent mobility/escape/chase
-Sick plays
-Good split push if team is doing poorly

Cons:
-Useless if behind
-AD Mid can cause trouble in full ad team comps
-Melee makes you easier to harass and farming more risky
-Late game is challenging
-Has to get close to waveclear
-Difficult to master

The following build has worked and gotten me excellent results in Diamond games.

Lucidity Boots -> Ghostblade -> BoRK -> Duskblade -> Last Whisper -> Maw (or Guardian Angel for final Item)

The reason this works is because QSS no longer removes Zed ult. All you need to do to kill an ADCarry with this build is R -> E+Auto -> Bork -> Ignite and the damage should be enough without even landing your Q. The full armor pen build is very devastating to squishy champs. This build often forces ADCs to go Steraks or Guardian Angel early, so be careful.