It is a good game. But for me it's too much. I wish I could complete the game 100% but there is just too much ground to cover. AND that you have to go under water which was a pain. A really nice and fun game, but could have been a bit smaller.
If the size of the game bothers you, don't even try Obilvion or Skyrim.. Those games are huge!
I don't own the game (yet), but I've watched my friend play, and it looks amazing.
I love Assassins Creed as is, but add owning your own pirate crew and ship vs ship battling.. I'm a happy man!