My wife got me a copy of Neverwinter Nights 2: Storm of Zehir for Christmas, so I've been playing for about a week now. Judging from the response on the official boards (http://nwn2forums.bioware.com/), people either love or hate this expansion. I'll try to give some of my insights into why this is the case by examining some of the new features in turn.
First and foremost, SoZ has a functional overland map of the sort that hasn't been seen in a major RPG release since the SSI Gold Box games. By "functional," I mean that unlike in the other campaigns, you don't just click on a location and insta-travel (tm) there. While new locations are gradually revealed on the map, you actually have to travel to them in real time (well, sort of--it's not like it actually takes you days of real-world time to go from one end of the map to the other). Some locations automatically appear on the map as you get close and some you will only see if you have a certain level of Search or Spot skill. You have an icon representing your party that you move around the map to various locations and frequently there will be random encounters that pop up on the map. These encounters also appear as an icon with a name and an encounter-level rating (this is equivalent to the "challenge rating" seen in the PnP game and actually in other parts of this game [which makes the two terms weirdly redundant]). If the EL rating is one or two numbers higher than the average level of the characters in your party, you probably want to stay away from that group, if at all possible, as they'll eat you for breakfast (possibly literally).
This segues nicely into one of the other new features, the party control system. In SoZ, you can have a party of up to four player characters and two NPCs. In dialogues with other non-player characters, you can use the conversation skills or attributes of any of the members of your party by clicking on a mini-portrait of that member and selecting the additional option(s). Your cleric may have insights due to a high Wisdom score or Heal skill; your wizard may have additional input based on his high Intelligence or Lore skill; any of your characters with Bluff, Diplomacy, or Intimidate may have other dialog choices too. You can import pre-made characters into SoZ or make your own. Any newly-rolled character starts at fourth level unless he or she belongs to one of the races with delayed levelling due to potent racial abilities (yuan-ti, drow, et cetera). There are a few new playable races, orc and yuan-ti, and a few new prestige classes, warpriest and doomguide of Kelemvor, but that was clearly not a focus of the expansion.
So far, I've found the experience to be quite addictive. The design decision to take a party-based approach and reduce the quest from something world-shaking to something more, um, region-shaking was bold and could have been disastrous. In fact, this probably brings me to my first negative point: the party AI. If you've played through the OC, you know the AI was pretty good in that campaign but if you played Mask of the Betrayer, you know that the AI was downright dumb. Party members would stand around casually as their fellows were slaughtered mere meters away; others would insist on changing weapons the minute you took your eyes off of them, frequently for no particularly good reason. Others would run around like headless chickens in combat. Well, sadly, all of these problems are still here in SoZ. Obsidian tried to get around these issues by incorporating a scaled-down version of a user mod, the TonyK's AI mod, into Storm of Zehir. Unfortunately, the scaled-down version doesn't work very well. Fortunately, the inimitable TonyK has kindly produced an updated version of the AI mod, which can be found here. I found MotB essentially unplayable without this mod and I think SoZ would be as well. As it stands, this experience reminds me of playing Gateway to the Savage Frontier and that's a good thing. It's fun running a party of your own creation through this kind of adventure. Mind you, it's not a cakewalk. You may find yourself in many situations where you're overmatched and your best option is to retreat (fortunately, sometimes that is an option). Also, it's pretty much not an option not to take a ranger; the Hide, Move Silently, and Survival skills all play a dramatic role in your ability to avoid hostile random encounters on the overland map, and, unfortunately, even with a ranger in your party, the map is frankly populated with entirely too many of these.
There is a new commodity-trading system which isn't too complicated and probably rewards careful use, but I haven't bothered with it too much. Also, crafting has once again been changed, and, as much time as I spent fiddling with it in MotB, it just doesn't seem to be too worthwhile here. I've always found the crafting system to be more trouble than it's worth in pretty much all of these games. Also, the death system has changed to a slightly less friendly approach that is somewhat more in keeping with the PnP system. Instead of everyone miraculously "waking up" at the end of a combat, you actually have to heal party members whose health is between 0 and negative (insert half of the number of max HP here) in order to revive them. If their HP dips below negative half of max (in other words, a character with 15 hp is reduced to -8 hp), that character is dead and must be raised (using Raise Dead or Resurrection spells or similar magic) either by your cleric or at a local temple (for a fee, of course). The consequence of this semi-permanent death is that you have to approach battles with a lot more care.
On the whole, it's a great "old-school" RPG experience of the sort we haven't seen in many years. Sure, there's an excessive number of random encounters and without mods your party members are as useful as an ostrich's wings, but the story is engaging and you'll want to keep building up your party so you can go back and finish that quest you weren't quite powerful enough to complete. Also, I can't close this article without pointing out how cool the cover art (with a viper wound through a skull) is--it's very cool!