To be perfectly honest this feels quite a poor and mind numbing expansion.
The two notable differences from previous NWN games is the Overland Travel and the ability to have up to 6 companions. Other than that it seems more like a Diablo clone than a proper NWN expansion. If you love repetitive Hack n Slash battles this game is for you, otherwise I personally wouldn't recommend it.
Overland Travel can get very boring, very quickly (although there is a small thrill in discovering a new location) as you have to physically move across the map (a la CnC Kane's Wrath or Dawn Of War), until later in the game there's no quick travel.
There's also not really that much of a storyline or plot, it mostly involves buying or finding resources and trading between different locations. Anyone expecting an epic storyline will be sadly disappointed especially regarding the end boss.
Anyone looking for detailed exploration and expecting to be able to wander through towns, villages and other locations will be annoyed to learn that whilst you can go anywhere on the overland map, the towns are all non explorable apart from the Temple of Waukeen, City Bazaar and Inn. Also many of the random locations seem cut and pasted, one location looks and feels very similar to another and there seems to be 3 different scenes for random fights in the middle of nowhere.
Crafting also seems to suffer, whereas in the original NWN2 you could have +5 weapons and armors and in MoTB you could have +8 equipment, in SoZ you are limited to +3 equipment.
Enemies also seem to have ridiculously low hitpoints, while the random encounters on the overland map are nearly all one of a small numbers of monsters (i.e. Skeletons, Giant Spiders, Stag/Bombardier Beetles, Wights, Will-O-Wisps, Shambling Mounds, Wyverns, Ogres, Batiri (a type of goblin) or Trolls).
Good points:
Couple of new classes.
Ability to have 6 party members.
Revisits old locations such as Neverwinter, Crossroad Keep etc.
Overland Map COULD be a good idea if there was more variety and a quick travel feature between places already discovered (similar to Fallout 3).
Bad Points:
Repetitive Hack n Slash encounters.
No real storyline.
No real incentive to level up or do side quests.
Crafting dumbed down (really +3 max? What's that about).
Repetitive overland travel.
End boss fight.
Buggy.
Broken quests.
Nowhere near enough RPG, more RTS style.