Well, I reckon that I'm about half-way through and I can safely say that Mask of the Betrayer is one of the biggest surprises of the year. I mean, it could have gone sideways for a lot of reasons, but the game is... really good.
The story for one is much more engaging that that of NWN 2 and picks up early on, so no more NWN 2 OC Act I (thank God for simple mercies). The companions are more fleshed out, and as a result the backstories are actually very cool to discover and listen to. Which is a great acomplishment in itself.
One of the things that I particularly like though is that MotB makes a lot more use of the D&D mythos than NWN 2 did, and because of it, the story feels... epic, but in the good sense of the notion. You're not fighting to save the world, you're fighting for your own survival, but interacting with the world and main characters on a much bigger level. Not to mention that it's not about you being the hallowed savior of the Forgotten Realms. Hell, after you reach the first city, you're a fscking time bomb and you're against the clock to save your life (although at first glance I would have to say that the Good oriented characters seem to have it better in regards to the whole Spirit Energy dynamic). Oh, and there's some "old-school" storytelling at work too (aka text-based descriptions, sometimes narrated), which is a nice touch and really manages to set the mood when it's used.
So yeah, if you hadn't guessed already, MotB is definetly worth buying, although it still could have done with some improvements regarding the interface and has some technical quirks (do
not set the Shadows to High, unless you want to see what a negative framerate looks like - everything else, graphics wise, is fair game).
Maybe in the next expansion, perhaps?