Just finished the campaign and sunk about 60 hours into this, so thought I'd post my review that I just put up on Metacritic.
The good news: Ubi actually listened to fans who wanted less hand-holding; more open ended assassinations. They even shelved the out of place if mechanically excellent sailing. Customization is also fantastic.
The Bad: The 3 main pillars of AC: Parkour, combat and stealth have always looked a LOT better than they actually played but in Unity this is taken to absurd levels.
Parkour in theory should allow the player more control with the added fast-descent, but a in practice only makes things more complicated compounded by the irregular Parisian architecture leading Arno jumping on things you don't want, refusing to climb or drop at all and getting stuck on objects constantly.
Stealth: I rejoiced when I saw we would finally have a crouch and cover system, believing they would simply rip the brilliant Watch Dogs version and use that. Actually, no. This has to be the most inconsistent, broken cover-system i've played since Mass Effect 1. Want to quickly get into cover? Well, my friend, press x to roll that dice,. because Arno may do as you ask.... or he may rather then going into the piece of cover you want go out of his way to slide into a doorway in full view of everyone. He may only go into cover if he's standing. Or crouching. Or he may automatically go from one of these stances to the other for no reason. He may go into cover nowhere near the corner, or more likely he will just refuse altogether. In classic AC fashion Ai, level design and guard placement and patrols are all over the place leading to the conclusion the devs have nary a clue of how to DO stealth. Combine this with the erratic, sticky Parkour and imprecise general movement and frustration exceeds blood-boiling levels,
Combat: Well it's harder - no REALLY this time. Unfortunately it's not for the right reasons. Sure they removed the press counter to literally set up a kill-streak to eviscerate every guard in the western hemisphere, and replaced it with a nice timed parry, and allowed guards to attack you together, but the delay in reponse to your commands means you will get killed through no fault of your own. And because the game automatically judges whether Arno is 'in' or 'out' of combat if you want to run good luck because you will have to fight the controls to force Arno out of the combat stance, which combined with the aforementioned delay means you will more often than not get impaled in the back by the time you get the game to respond to what you want. Batman Arkham this ain't.
Playing this game is one of the most heart-breaking and frustrating experiences I've had in gaming, because it's clear Ubisoft really listened to what people wanted and clearly tried to produce the ultimate AC - a game that SHOULD have been the pinnacle of the series. When things work the game feels beautiful. Unfortunately that is the problem. Things work about 20% of the time. For every time Arno balletically glides over a ledge and slides down a rooftop into cover, before seemlessly wrentching a guard round a corner to sever his carotid artery without alerting a soul to his presence there will be another 4 where he'll get stuck on the ledge, or jump off the rooftop, or refuse to go into cover and get seen. It seems a combination of lack of talent to implement consistant, responsive controls in favour of their beloved animations and the usual Ubisoft rush-to-shelves-for-holiday-season have given us a beautiful yet horribly buggy, unoptomized, broken mess.