Black Flag is a fantastic all round game that has introduced some new elements to the AC series in a fantastic way, got rid of the dead wood from AC3 and brought us a protagonist you not only care about but enjoy playing with.
The new more open world map is a fantastic change that has got rid of the dreary repeated towns of AC1and2, Expanded on the outdoors and open seas introduces in AC3, and removed the IWIN function from Brotherhood (The ability to call in a swarm of Assassins to make tough missions a free win).
It has it's flaws. It has a few more bugs and glitches then I remember in any of the previous games and it's still far to easy. Combat is no longer a struggle which is disappointing. You either dodge and attack, break defence and attack or just shoot everyone. If that doesn't work just stand there, drop a smoke bomb and kill everyone for free.
Putting that aside though, it's a fantastic game that will eat up hours of your life.