1UP has published one of the first online reviews of Neverwinter Nights 2: Storm of Zehir, and, for the most part, they seem to have liked the expansion. The score is a solid "B+", with a few issues pointed out at the end of the article:
But like all of the games saddled with the buggy NWN2 engine, Storm of Zehir has its problems. Besides a rather lackluster story, the overland map does a few weird things as well. For starters, every time you go from an encounter to the map, all of your long-duration spells expire. My 24-hour-duration spells shouldn't fizzle out this quickly. Coupled with the game's inability to rest without risking a chance encounter (you may safely rest in towns), this really nerfs spellcasters (I hope one of NWN2's modders comes up with a fix, if the developers don't address this in a patch). And while the conversation and crafting systems allow you to use all of your parties' skills, the overland map oddly restricts you to just your selected character's abilities -- a weird step back, given the freedom of the party system.
Storm of Zehir may have a lackluster story, and that's OK, because it doesn't need one. It gives you the means -- the freedom -- to make your own story. And isn't that why we like RPGs in the first place?