In my humble opinion, it was great, and even better than the original
There were certain problems that I found in NW2 that made it less enjoyable. For one, the character interactions seemed a 'bit awkward'; the story also, in my opinion, had an extremely lackluster ending. But fear not, because I felt that MoB did an excellent job tying up loose ends. First off, there's the convenient option of importing your character from the original campaign, which increases the immersiveness of the overall experience. MoB, overall, just felt like a much-needed "patch" for the original Neverwinter Nights 2 game. The story not only continues, but in my opinion, improves and develops.
There are less party members this time around, and scripted events seemed much more 'personal'. Characters have more interesting backstories, and really, I just felt more 'connected to them'. Gameplay is also enjoyable because all the 'annoying events' that did occur in NW2 were removed. No more disorganized final boss fight. Less frustration; more personal power (since you do enter epic levels). The game, overall, just feels 'tighter'. The storyline is also fantastic. MoB creates a dark and personal ambiance that serves to strike chords within the player. The choices, are also highly impacting.
Despite its greatness, MoB is not perfect. As others have mentioned, the spirit hunger can sometimes bug you (though I honestly didn't find it -that- bad). Also, many of the issues from the original NW2, (understandable since this is an EP), transferred over. Camera angle; general visual-attractiveness; occasional moments of 'blandness'.
In short, MoB is a great game that, in my opinion, improves greatly upon the original NW2 experience by providing better characters and a deeper/more satisfying story (that also, closes finely via a spectacular ending). If you liked NW2, I highly suggest that you give this expansion pack a try. It feels like it's more than just an expansion, since it ties up loose ends in the original campaign, that really, just felt oh-so-unfinished. And honestly, the storyline this time around, was just fantastic.