Yeah, I am loving this. So much so that I have the urge to do everything. Every little side mission, renovation, guild challenge, flag, feather, the works. For me it basically keeps the good bits of two (lots to do, involving world, bonkers story, likeable characters, lairs, subject 16 puzzles, Jesper Kyd score) and adds lots on top. I feel the Borgia Towers, Brotherhood, execution streaks and War Machines (though I like reaching them more than using them) are all great additions, and it's nice that you can now replay missions. Am also quite fond of the 100% synch mechanic, as it forces you to do certain things that you might otherwise not have attempted. Definitely adds a bit of challenge as well. Parachute and crossbow in particular are ace new itemes, the latter is fantastic if you want to play more stealthy.
While you could argue that the best missions are actually the optional ones (Borgia Towers, War Machines, Lairs) I am quite enjoying the story missions as well. To me it seems they give you more choice than the ones in II and I like the variety.
Rome is a great city as well, it looks nice and there's lots to do, but it does have some technical issues that I didn't encounter in II.