It's worth purchasing for the multiplayer and the user created content, but the campaign mode is mediocre at best.
Of course, this community doesn't thrive on multiplayer alone, after all, we are not talking about an MMO. This brings me to the game's other strong point: user created content. Yes, the game comes with a powerful editor (the Aurora Toolset) which gives users the opportunity to create their own campaigns. It's pretty easy to use and the results can be impressive. And because of this there are many good modules available for download, which considerably extends NWN's lasting appeal.
But, although I haven't read any statistics, I'm willing to bet that a large number of people who purchased this game did so for the single-player campaign alone. And in this regard, NWN does not live up to expectations. Why? Because its story fails to impress, it's just forgettable. Bioware games always have something about them, a certain quality that makes them fun, NWN just doesn't have that. Maybe it was because of the forgettable story, maybe because of the two dimensional characters, or maybe the bland dialogs and generally bad writing, or the boring quests... ultimately it doesn't matter why. It just doesn't have that magic that made Baldur's Gate and KotOR so good.
Of course, being a D&D 3E game, NWN has tons of options on how to create (and further develop) your hero. Your choice in this regard can alter gameplay to a certain extent, but do not expect to encounter too many significant choices along the way. Another problem that NWN has is its difficulty. A competent build will storm through the game without encountering difficulties, there are only a select few battles that are actually difficult. There are a few puzzles along the way that break down the gameplay quite well... but they're rare. It's also disappointing how little control you've got over your henchman. Oh yes, I forgot to mention that. NWN is not a party-based RPG, like Baldur's Gate was, you only get to take one henchman with you, and you can't control him directly.
On the tech side, things aren't great. The Aurora engine looks dated and the game takes disappointingly little advantage of the 3D engine that powers it. Particle and spell effects look fairly good and character animations are great, but ultimately, having 3 dimensions helped little. The user interface is excellent though, it makes interactions easy and it also looks good. The inventory system is especially well done, it's based on a grid system with tabs to keep your loot organized. I also want to mention the camera which can be a bit of pain at first... but the fact that you can change your perspective whenever you like is great. I usually used the top-down mode and the game looks as if it were made with this view in mind. I'll wrap this up by mentioning the sound: NWN isn't all that great in this regard, it feels generic for the most part, specially the ambient sounds. The musical score will capture your attention once in while but it's nothing spectacular... and the constant repetition of the same themes doesn't help either.
So overall, Neverwinter Nights is a great purchase for people looking beyond the official campaign. And it's an accessible game, newcomers to D&D and RPGs in general should take it into consideration because it can be an "educational" experience.