Assassin's Creed only gets good when it stops holding your hand, which is unfortunately half way through the game. But at least during that first half, you get ship battles.
Once it lets go, it really is fun to figure out how to kill your target; whether that means killing everyone and everything in your path with well executed timing or killing the fewest amount of people with excellent timing and cunning.
I'd say get it if you liked the other games.
Haven't played multiplayer but it looks interesting.