Spent a little time with this and I am VERY impressed so far.
Graphics: Possibly the best I've ever seen taking into account the open world setting. In motion everything looks great. If you stop and look closely at things it looks even better. 100's of NPCs walking around with full cloth physics on each one of them. Textures on the most random objects are very good. If you look in the distance while running full speed you, sure you can see pop-in sometimes, but for the most part it's not noticeable when you are focusing on your character and immediate surroundings. Also, the framerate issues that everybody freaked out about appear gone via patches. I've only had 2 instances of some stutter. Outside of that I couldn't tell if the game is a solid 30 FPS or not... it looks good that's all I know. Long story short, the graphics in this game are exactly what I was looking for with Next Gen... looking forward to how they can improve going forward.
Gameplay: So far so good. The new climbing and climbing down mechanics are the best yet in AC games. The going down part is especially awesome and has eliminated the need for them to have random carts of hay on every block. Typical AC problems still exist where you character doesn't always do what you want.
Story/Setting: Story is typical AC, but much like Black Flag the setting is so awesome that I'm really drawn in. In this case, the outstanding graphics really help too. The countless # of NPCs and they way they react to situations as a crowd really sets this one apart from other AC games. And I've only been into one of those "time ripples" or whatever, but that is really awesome too.
The only negative experience had was the one time I tried co-op. Teamed up with a random and at one point it was literally broken with guys standing around, swords drawn, but nothing was happening. Not sure if it was lag or what, but probably more online issues that seem to plague next gen games!