Zac is by far the most fun champion I have played! He is not necessarily the strongest, though his ability to leap over walls and gank from just about any where he wants is amazing! Zac has three forms of CC, he is tanky, AND has fair damage! These factors among many others make him a huge threat to any enemy team. Also, Zac is very comparable to Amumu but easier to play. Since Amumu is almost always banned currently, it stands to reason that a champion who is very similar to Amumu would be a popular future pick for tournament play!

Pros

Strong CC
Strong Ganks
Strong Engager
Fast Clear
Good Sustain
Almost never gets invaded early game
Passive allows Zac to bait quite well

Cons

Elastic Slingshot is interrupt-able
Likely to be a top ban pick in the future


Note: Ive read many of the other guides con's list for Zac.. and I think to myself SERIOUSLY?! For example "Enemies can kill you while you are trying to reform after death" ... The fact that you even get that 2nd chance is a PRO not a con!! Losing health in fights? You technically GAIN health during fights because even though your abilities cost 4% of current health you are constantly picking up 4% of your max health even if you dont try to! So really I cant say much for cons yet on Zac... Most of the cons are not even really cons... Another example Elastic Slingshot is interrupt-able.. Yes it is, and that sucks but the fact you even have such a great spell is a pro to begin with. I will go ahead and list the closest things to cons I can find though...

Mostly standard runes except you may not like the scaling magic resist runes like I do. Zac does a fair amount of damage, so I do not elect to go pure defensive..

Everyone has their own preference when it comes to which defensive masteries. This is my personal preference on masteries. I don't get relentless because I use boots of swiftness, and I dont get Legendary Armor because I build mostly AP/Health.

Flash : Due to Zac's slingshot ability being able to leap over walls to escape or chase with, I considered getting ghost instead.. However the flash also works very well with Zac's ulti to make sure you land the knock up on players.
Smite : Standard for most junglers. Additionally smite helps you counter jungle, stop enemy players from stealing your buffs, and helps snag barron/dragon a little faster.

I hear many people say that Zac is weak early game. I have to disagree. Any stronger and he would be ridiculously over powered and be a guaranteed ban. [Zac] may not have huge damage early on but he does have a small stun, effective slow, and ability to leap in from just about anywhere giving him the ability to pull of some very effective ganks at level 3. Further more, Zac's passive allows him to come back after dieing! So you can gank very hard and as long as your team gets the kill you can come back. As long as I have one or more allies near me (enough to protect my pieces when I die) I will bait to get first blood. The enemy team will often be to aggressive and sacrifice themselves to kill me, which does them no good because they are not in a position to finish off my pieces!! (Granted, better players will not take this bait.. I felt it worthy of saying because it happens so often!) And once Zac reaches level 6 he has nearly guaranteed ganks with two stuns and a slow, and very adequate damage. Especially when ganking bottom lane for a double kill!

When ganking you should have a fairly good idea of where wards will be.. So simply gank from any other angle!

Zac's Unstable Matter can be spammed during his ulti and should pretty much be used on cool down for most any fight. My standard engage is Elastic Slingshot into Unstable Matter almost instantly when you land on your target(s), followed by Let's Bounce, and of course spam Unstable Matter on cool down. Its important to use Unstable Matter before your ulti because your ulti will knock them out of range of Unstable Matter.

Always remember that Zac's passive heals 4% of your maximum health and the cost is 4% of your current health. So if you wait until you are lower in health to use the potions your potions will go a longer way. I typically use the potions as I am hitting level 3 so that I am near full health for my first gank.

Let's Bounce will knock enemies away from you. So use it to knock enemies toward your turrets and team mates, or to knock them away from their turrets and team mates.

Fly by Smite steal! http://www.youtube.com/watch?v=GGcWE4Aq_kU

Zac's Elastic Slingshot can be used quickly without charging it as an immediate interrupt, small stun, or just to deal that extra damage that can mean the difference in winning a fight thats very close.

Vi Medium/Hardis one of the few counters to Zac. Her ulti counters your ulti. I have not really found most Vi's to actually be hard but of everyone, Vi is probably one of the harder opponents for Zac.

Renekton Medium-easy In general, renekton is not very hard. He can be hard for top lane maybe but this is a jungle guide.

Amumu Easy You and amumu are VERY similar to each other! You are both tanky and stack health.. the main difference is you do more damage.

Cho'Gath Medium-hard This can be a relatively challenging opponent if they are good. Im not going to go as far as to say hard.. But of the possible opponents this is one of the more challenging.

Xin Zhao Medium As long as you get a good start (aka successful ganks early) then Xin is not much of a threat, but if you did not get a good start you will probably want to wait until mid game to take on Xin alone.

Nocturne Annoying A good nocturne will be right there with you for many of your ganks.. You jump in and naturally Nocturne will be close behind!

Everyone else is mostly easy.. If you disagree please do not hesitate to comment with who you find difficult!

Zac is very strong and even more fun. I would not call him "OP", however he does have a large amount of influence on games due to his massive area damage and crowd control. Zac is a new champion, so there will be plenty more information for me to add about him as time goes by. My in game name is OCD2 if you have any questions about Zac.