A great game with a few annoyances that are easily overlooked.
This expansion pack features some new classes, spells and so on, which bring a nice additions to an already complex game, making it worth it to go through the single player of the original NW2. There are some drawbacks however, you'll notice that the fights you get into are either incredibly easy or insanely hard at some points.
I've heard also people complaining about the spirit eating mechanic and, yes, it IS very annoying the first time through (certainly annoyed me). But i noticed at my second and third runs through the game that once you know how it works it's not big deal, I even came to like it as it forced me to make more strategic choices concerning the spells I used, the class or multi-classing i followed and most importantly when to rest. During my first NW2 run, i used the rest mechanic every one or two fights, this forced me to drop those habits that eventually spoiled the fun of the game, making it too easy.
However this mechanic can really be either annoying, hateful or just plain fun, but regardless you shouldn't let this spoil such a great game.
During the course of the game you'll get far fewer companions than you got in the original NW2 which is positive in some ways as the game often forced you to take some characters with you and it could make some parts hard if your main character was the wrong kind of character. You'll also noticed it's much easier to get attached to these characters as you can learn to know them much better (besides who can't love the mighty rainbow bear). And you're actually rewarded for doing this as getting enough influence gives you feats.
The storyline is also more satisfying than the previous game as it's much darker and murkier, you really get the feeling you're caught in something vital.
All in all this is a great game that shouldn't be missed by any who like the original NW2.