YouGamers.com has posted a comprehensive review of Mask of the Betrayer. With a three of five star 77/100 rating, the reviewer said:

Mask of the Betrayer is a chilling, almost horror-like adventure, but there are very few opportunities for chillin' in it. Your terrible affliction haunts you most of the way, and you'll have to be at the top of your game to beat the odds stacked against you. In practice, this means a lot of effort and quite a bit of headache trying to figure out the best items, weapons and spells for your character and companions. On the other hand, an uber careful character build makes it possible to waltz through the game, soloing every combat (or so I'm told).

Neverwinter Nights 2 has improved a lot in one year, but it still doesn't reach the multiplayer capabilities or popularity that the original Neverwinter Nights enjoyed in its hayday. Hopefully Obsidian Entertainment takes the lessons learned from NWN2 to heart, and the next expansion redeems the title in even the hardcore fans' eyes. Those who have stuck with NWN2 all this way are truly hardcore, and Mask of the Betrayer is a solid expansion for any fan of the series.

Read the rest and view a gameplay video at YouGamers.