NWN: Hordes of the Udnerdark tell a great story and adds much content to the NWN series even though it lack polish.
There are parts of this campaign that are so well done it immerses you into the environment and storyline. The story line is well written an easy to enjoy especially for a fan of Forgotten Realms but, some of the puzzles within the campaign are rather frustrating making the game much harder than it should have been. At one point in time there is a puzzle that you need to take a ring on and off to allow movement through a series of magical barriers. That may be easy enough to do but, figuring this out is another story. I did not find any clues indicating I was supposed to use a ring, nor does the game ever notifies you that this ring was placed in your inventory. (Maybe this is a glitch having to do with the age of the game. I am playing this on windows 7 and there have been quite a few errors.) Some of the puzzles towards the end of the game feel like the developer ran out ideas and put little effort into them. They are far more tedious than fun. This is probably the biggest downfall of this expansion. The campaign is rather linear in terms of game play but that doesn't inhibit the story from being well written.
Party mechanics are still not the best in the world; they could have really used an upgrade in this expansion. The dialogue and character development that are present seems moderately rich but lacks choice. The lack of choices can be overlooked at times though due to the lengthy and descriptive dialogue. Sound effects and game music is very well done as were the graphics at the time of this game's release. The music that goes along with the final boss fight is incredibly awesome; Jeremy Soule does an incredible job. This expansion added a great deal to the tile sets in addition to the character content, this truly helped to develop the online community of NWN releasing hundreds upon hundreds of mods for the game. Servers like Howard's Lord of the Rings mimicked MMORPGs but with extreme customizability. (The first "real" mmorpg I played.)
For me, the story line and content outshined the mishaps I experienced playing the game. If you are a forgotten realms fan and haven't played through this expansion it could definitely cure for boredom. Heads up though, this game can be glitchy due to its age.