Zac rework thoughts from a Zac main
Zac has been by far my most favorite champion. He's a bouncy, tanky, bursty, CC hardened Jell-O maniac with lots of maneuverability that most other tanks can't compare to; and his rework is no different concept-wise.
When I first picked him up, I didn't change anything runes or mastery wise to see how he compared. To my surprise, he cleared slower and had lower HP coming out of the jungle after his first clear. I had to be careful trying for a second or third round of monsters, unlike old Zac. I wasn't sure if it was his damage or tankyness that got hit hardest, but there was definitely a difference.
After a few games I changed up his runes, little less armor and more flat HP, and he ended up with a much safer clear, similar to the old Zac.
Abilities
His Q has become a bit more unreliable and hard to pull off. I wasn't too excited about it other than the occasional Mike Tyson's "now kith," gimmick I could say over Skype. I think the reason it's so unreliable is that the second hit needed is a regular auto attack range, making the distance needed after a long Q feel very...off.
W isn't really different, but I started to level this after maxing E.
E is still his bread and butter ability, being reliable for stealth ganks, great escapes, engages, and epic monster steals. While it doesn't feel as rewarding as it used to landing that 4-man knock up, I'm not going to complain much. It's still a great ability.
R is where I hesitate. From a developer standpoint, I understand it creates interactive gameplay for everyone, even allies. Mechanically, you can pull off great combos by pulling in the squishes for easy kills, separating the tanks by isolating them behind a wall, or forcing a teamfight in your advantage.
What I DON'T like about it, though, is that the ability feels slow to the Zac player. It's not a brain dead ability, either (And I'm both happy and glad it's not) where you E in behind the adc or mage, and knock them towards the ally team. You now have to figure out if they already used their mobility, if you can land the knock up just right, and pray your target (s) can't escape or walk out before you're able to actually pick them up. There's a LOT more to consider and prepare for now, maybe too much, and that's why it feels so slow and hard to properly pull off.
(Don't get me wrong, it can feel EXTREMELY satisfying if you pull off that perfect bounce).
Builds
He's still my go-to tank, but recently I've been having much more success using him as a tanky AP champion. If my team has a dedicated beef cake, I go tanky AP because I just feel it does the job better and greatly synergizeswith his new kit.
With just the mage jungle item, Rylai's, and Liandry's Torment (With a late game Warmogs, and the MR/Armor boots after Rylai's) you really pack a punch to squishy champions. Hell, you can kill or deal 2/3 HP to an adc with just an E-W-Q combo. You're also still tanky enough (with the right runes/masteries) to pull off that E-R bounce and survive like you would full tank.
Tl;dr he's a different monster now. Even with just two ability changes, he's gone from straightforward combos to a more complex game changer. Q feels awkward and ineffective, and R feels slow but interactive.