Mysteries of Westgate is not really an expansion pack, even if the Wikipedia article claims otherwise. It has nothing to do with the original Neverwinter Nights 2 campaign or the two expansions. Nor does it add an expansion's worth of new stuff to the game. Merely few monsters, a sewer tileset, and some new music.
MoW takes you to the city of Westgate, where your character wishes to find information about the mysterious domino mask he/she has somehow acquired and which apparently can't be simply discarded. The motivation to get rid of the mask is questionable, though; it does not actually do anything when carried in the inventory. Equipping it summons a spectre that slowly damages you but as you rarely need to do so (even when the mask gains couple beneficial powers later on), all it does is take one inventory slot while you are in no hurry to get rid of it.
The city's main areas can explored freely but doing so beforehand is not worthwhile most of the time as the majority of doors are quest-locked. The way side quests are found while following the main one is quite brilliant, though. You do not really need to make an effort to find them. Although, I may have missed one quest as one chest in Undergate was left unopened.
I would have been a whole lot more suspicious about how easy the fight against the Night Masks' leader was if I had finished Elminster Enraged before playing the module. In the novel, Elminster mentions that Manshoon ruled Westgate as Orbakh before his current activities. This is not revelead in MoW, as far as I know. My party certainly never would have stood a chance against the freaking Manshoon if he did not want it to seem so. I cannot say I was surprised to see Orbakh reappearing at the end to talk to my character regardless. I was kind of expecting everything at that point.
The module advertises it to be easy enough to be suitable for all kinds of characters. And it indeed was not particularly challenging. One side quest's last battle against three giant spiders and a dude proved quite difficult, though, and I had to cheese it by retreating back to the entrance and defeat the spiders one by one. I might have had easier time if I had found the last party member before going there.
I had never used the parry skill before and thought that maybe a class/prestige class build around it would be a good choice for using parry. And what is more duelist than a rapier and a dagger. I probably should have done a little bit more research, however. Around level 13 or so (you start the module at level 8) when I took the 5th level of duelist, I found out that Precise Strike cannot be used while dual-wielding.
I had to the check the Dungeon Master's Guide if this was also the case in the actual rules, and apparently it is. Although in PnP you can simply choose to not attack with your off-hand weapon to gain the damage bonus. NWN2 of course is not that sophisticated; Precise Strike simply is not active as long as you are wielding two weapons. And if I had unequipped the dagger, I would have lost all the parry bonuses feats were granting me.
I wonder if I should check out the premium modules that came with the first game next. Going back to NWN's engine might be difficult after the playing the second game, though. I guess I could also try the Icewind Dale campaign for NWN2. See if it compares to Baldur's Gate Reloaded.