I have an xbox one! I have not played Black Flag yet, because I was waiting for it to be added to the game library for EA access. I'm not sure if you are aware or not, but if you purchase the license for EA access you get access to a load of games included in EA access back catalogue - it's actually a pretty sweet deal, here is the blurb -
'EA Access lets you play more games for less. Binge on EA's best Xbox One games, including Plants vs. Zombies: Garden Warfare, FIFA 14, Battlefield 4, Need for Speed Rivals, Peggle 2, and Madden NFL 25, with more titles being added over time. That's more than $200 worth of games for just $4.99 a month. Play as much as you want—you’ll never run out of fun.'
I believe you can pay annually and it costs even less, for instance, it's £3.99 a month here, or £20 for the year. And I think you can cancel it whenever you want, but you probably do not get reimbursed what you already paid.
Also, if you get xbox live Gold, you get 1 or 2 free games a month with that. Other free games right now would be Warfame and Pinball.
Just wanted to throw that all out there, as I know gaming can become expensive quickly!
Currently I'm playing Halo, which is bad-ass as it's all 4 Halo games on one disc :)
Enjoy Assassins Creed :)