Warning: There will be minor spoilers as a fairly important game mechanic is added partway into the game.
Let me start off by saying that my experiences with the original campaign weren't entirely pleasant. The plot took forever to get anywhere, I didn't like any of the party members, and the bugs, oh god the bugs! It was a little over a year ago that I played through the OC, and even that long after release there was one spot where I had to go online and look up how to go into the command console and trigger an event that wasn't working (and thus wouldn't let me advance the story). Maybe this has been fixed since then, maybe it hasn't. God knows after installing MoB and Storm of Zehir, the game spent somewhere around an hour and a half downloading and installing patches, so maybe that part was fixed. I mention this for two reasons though. The first is because it's hard to review an expansion without mentioning the base product, especially since in and of itself, I couldn't in good conscience actually recommend just the base NWN2 game to anybody. The second is that people reading this understand that a big enough fan of these kinds of games that it grants me a nigh infinite amount of patience with them as evidenced by the fact that I managed to finish the OC and then eventually bought the expansions rather than giving up and uninstalling. Take that into account when reading the following review.
To start with, I'm not a complete sucker. I wouldn't have bought the expansions if they hadn't received positive reviews. From what I can tell, I'm maybe halfway through, and from what I've heard it's not particularly long. The plot still isn't great, but you get a pretty diverse party this time around including a Red Wizard of Thay, and a Hagspawn among others. The story is pretty linear so far, and there doesn't seem to be too many side quests, which is a little disappointing to me. People skipping over the OC and going straight to MoB need not worry too much about missing out on anything important story-wise. While you might not understand some of the references they throw out here and there, it's not vital to understanding MoB's plot.
The spirit devouring mechanic introduced about a third of the way into the game is essentially this game's good/evil mechanic. Essentially you've got, something... inside you with a hunger for spirits. You can either try to keep is suppressed and over time the hunger won't be as much of a problem, or you can keep it fed by absorbing various spirits and elementals you fight along the way. There's also a few story points where this will come into play that result in getting different powers or party members. For people who like to play evil, it's certainly viable here, and might even be the superior choice.
The main problem I have with this expansion is the gameplay. New characters start out at lv. 18, and people bringing in characters from the OC might even be higher (I hit lv. 21 partway into the first area). This means we're dealing with Epic level D&D rules, which everyone knows are balanced for shit. The main challenge here is going through the effort of buffing up each time. It hasn't been too hard finding places to rest so far, and a well rested party with all their spells handy can mop the floor with just about anything they come up against. They challenge comes in only when you're too lazy to rest and rebuff after every battle and you try to see how long you can go before resting again. The hunger meter also discourages constant resting to extent, but I've found that to be fairly easy to mitigate.
I haven't come up against any bugs yet, aside from the occasional minor graphical glitch.
So then, to buy or not to buy? Depends on a variety of factors:
If you already have NWN2, you can just pick up MoB for around $10 now, in which case: Buy it!
If you don't have NWN2... well, it's still pretty cheap (you could either pick up Gold for close to $30 or you can probably get them separately for around $20 total). On top of that Storm of Zehir (which I'll review next) is only around $13 right now, making a pretty good overall package for a relatively cheap price. Since you can't rent or demo this, I'm giving it a Buy it! rating for the harder core fans of the genre, but Avoid it! for pretty much everyone else.