This is such an odd one for me, because I love the Assassin's Creed series, but really struggle to tell you why I enjoyed this game so much. The fact is, this is not an incredibly well made game. It's certainly a good game, and a lot of care has obviously been put into it, but it's just not quite up to the quality of either AC II or AC Brotherhood. This really does feel like a churning of the same formula just to get a 2011 Assassin's Creed title published.
As aforementioned, I did have a lot of fun playing through the story, but this was most probably due to my love of the gameplay and progression. When it comes down to it, the story is rather filler, and the gameplay elements that they have added are pretty boring. The Den Defense mechanic is frustrating and dull, and the first person Desmond sequences are ridiculously awful.
In regard to whether the game is worth playing, it really depends on how enthused you are by the series. If you love Assassin's Creed, chances are that in the 7 months since it was released you have already played it. However, if you're not too bothered, then you've done right in not picking it up. I definitely don't regret purchasing and playing it, but it must be said that the variation promised by Assassin's Creed III is more crucial now than ever before.