Took roughly 40 hours but I got it! Good thing is that most of them were actually really enjoyable, you get to experience the already awesome atmosphere and world in more depth than if you just focused on the main campaign. But that's not to say it didn't have its share of obnoxious achievements and challenges.
Awesome game though, this is exactly what the series needed. AC1 was amazing for the time that it came out despite being heavily flawed. AC2 was almost perfect, definitely in my top 5 games of all time. AC: Brotherhood was a ton of fun but significantly less exciting because it just felt way too similar to AC2. AC: Revelations followed that trend too and made it even less interesting than before, on top of that the conclusion to a character as awesome as Ezio was such a letdown.
This game was a huge breath of fresh air. New time period, new location, new characters, new ideas like the naval missions, etc. Even the multiplayer feels really polished.
I would agree except for the fact that the game's ending, at least in "present day" is absolutely terrible. They pulled a "Mass Effect 3" and basically gave you a thrown together jumble that made little sense, gave you no real choice in anything, and wasn't really faithful to the themes of the earlier games.
Other than that though, the game was amazing, and unlike ME3 I'm hoping there will be future games to remedy/build upon where this one left off instead of making us accept it at face value.