I liked it too. As DN said, it is like AC with less polish. The gun fights are decent because it's not focused on shooting, although you will do plenty of it. The sneaking can be good but it's not great. The acrobatics (climbing, jumping, whatnot) can be a little finicky and don't feel as good as AC but it's still enjoyable.
The missions are not as repetitive as AC. Each mission will have a distinct bit of story to go with it. The world is open, I don't think you need to unlock areas but, obviously, you wont have authorization to enter restricted areas.
Beyond the story missions, there are a lot of free roam missions which basically consist of blowing up a zillion nazi installations. I had a few weeks where I wasn't working and all I did was play this game. By the time I finished all of the free roam missions I was completely sick of the game. The free roam missions are repetitive because it is just blowing shit up with no real story but it is not necessary to play or enjoy the game.
