Cutting straight to the point.
Xin is a really strong early game champ and this strength is disgustingly abusable in early game ganks or jungle invades.
Xin's lategame is also very strong and you can build a variety of items ranging from auras to flat hp to flat damage and make use of them all.
Xin's a really easy champ to learn and can carry games even with 4 ******s on the team which is always helpful.

Armour seals and caling Magic Resist glyphs are pretty much staple.

For marks you can take flat AD, armour pen or flat attack speed.

For quints take either: flat AD, armour pen, flat attack speed or move speed. I'm also gonna try out lifesteal quints.

21/9/0 and 9/21/0 are good. Definately run 21 in defence if your team is ******ed and you're the only melee.

If i'm running move speed quints I like to run 21//6/3 with the 3 points into wanderer. You'll find that the extra move speed from these runes and masteries + boots 5 means the enemy has to ward REALLY far out to avoid a gank. If you have a Janna on the team the movespeed is actually just broken.

Doesn't really matter, you're Xin Zhao, just press buttons and people die.

Spells are good, you need them. ;)
Don't forget to use your ulti on the engage as it deals a **** tonne of damage and makes you tanky as all hell.

Pros:
Lots of base damage.
Built in sustain.
3 forms of CC.
Armour shred.
Can build lots of different items.
Gap closer.
Low cooldowns.
Naturaly tanky from ulti.
Cons:
Hmmmmmmm, countered hard by a broken and fairly popular champion: Janna
Ezreal is also a douchebag.

Meh, soloqueue players are ******s and that's why you're playing Xin anyway.
Just make sure your team is around to follow up on anything you do and go ham, take all the kills you can since jungle farm is pretty ****. **** your carry anyway you're the one that has to take on the enemy carry right?

Don't pick Xin into Janna -
Gank people whenever possible and as early as you can.
Invade the enemy jungle if the enemy jungler is weak and you think your mid isnt completely ******ed for some free kills.
Carry the game n get dat elo.

Also dont forget when you get trolled in solo queue that Xin can literally be good in every lane even as support (sit in bush till you get an opportunity to one shot the enemy adc).