I like it just as much as I liked AC2... basically it's pretty much an expansion to AC2 than anything else even though it does have some new elements and some slight gameplay tweaks... kind of what Mario Galaxy 2 is to the first. That's not a bad thing, it's a very good game and there's a ****tonne of stuff to do in it.
And yes... it's essential to the AC story. Don't be fooled, just because it doesn't have 3 at the end doesn't mean this is something you should miss out on because you may think it's a side story... it's not. It is essential. *nods*