Yasuo is in my humble opinion one of strongest and best champions introduced in this game, not only does he shine on mid, but he can also be played top if you are acustomed to him.

He is melee ad curier (He is not marksman, for people out there that will say my guide told me to go bot), which means he can easily carry his team to victory if player has skill and he will become really useful late in game.
When to pick Yasuo
-When you have someone who is tanky in your team(support doesn't count, only Leona)
-When enemy team does not have lot of cc's(You can deal with them by shifting your build, but better be safe
-When you are sure you can win lane with him(Countering someone is key to winning game, so by taking Yasuo on a lane that you counter, you will make it easy for your team and your jungler to do their part of the job
When you shouldn't take him[/b]
-When you cant win your lane with him(This is obvious, but i have to stress this out, if you can get fed even countered, pick him and don't worry)
-When they have tons of cc.

Key to being successful with Yasuo is mainly up to you, but sometimes if you have nice team that can help also. You are initiator in late game(or after your randuin), before that any mindless running in huge croud is most likely suicide(if you are fed its whole other story, but its mainly on your resolve).One key to surely rocking game with Yasuo is having lot of knock ups, which means landing your 3rd Steel Tempest or to have Malphite or Wukong do it for you.
In my opinion Wukong, Malphite, Riven and Jarvan IV are your best friends.

Pros
- Survivabillity
-Mobility(Some of you will say its same as survivabillity, it is not)
-High damage
-Passive op
-Knock up+ulty=USEFUL AS HELL
-Crit benefits
Cons
-Squishy early

This build on top will bring full potencional of Yasuo, and i always play this on him its not that i find it good for my style, these are just items which he needs.
Someone will say why i didn't put any armor penetration there, well that's simple just look what his ulty gives you for 15 seconds and it will be clear to you as a day.Now someone is wondering my Blade of the Ruined King is better than BT, its not mainly cause BT gives more damage and Life steal.Only item i will explain is Trinity Force.Trinity Force gives you everything you need as Yasuo:
1st You will get crit
2nd It helps with your passive drastically
3rd Improves everything(it is sometimes overpriced,but trust me its bred and butter for Yasuo)

This is all i can tell you. I hope this guide will be helpful.As it is my first guide leave me your comments(good and bad ones).If you need more informations feel free to ask.