Good if you enjoy crafting game worlds, awful if you're looking for a solid single player experience.
Neverwinter Nights is set in the Forgotten Realms city of Neverwinter, and the regions around it. You assume the role of an adventurer attending an academy in the city (sounds too Harry Potterish for my liking) as a "special student". Apparently you're more uber radical than everyone else, though it's never actually stated why. I mean, you're level 1, with no experience; how could you possibly be better than Mister Tickles the level 1 bard?
The storyline of Neverwinter Nights is hilariously bad, but I don't consider Bioware the epitome of great story telling like some people in the first place. Essentially, the story just involves your character going on a bunch of Fed-Ex quests to battle evil forces that are brewing in the north. Everyone knows that Fed-Ex truck drivers battle evil frequently, so the story is perfectly believable, yes sir.
The gameplay doesn't differ much from other D&D games. It uses the D&D 3 rule set, but since I have no idea what the difference is between D&D 2 and D&D 3 I find myself not caring. You can create your character from a variety of race, and class combinations, and as you gain experience and level up you can multi-class them. Hooray for Fighter Bards. NWN also comes with an easy to use toolset that lets you create your own modules for your friends to laugh at. While it's easy to use, the toolset also lacks freedom, which was a problem that was thankfully fixed in Neverwinter Nights 2's toolset. There's also a multiplayer component, but I'm not cool enough to hang out with the 1337 dewds and their +5 haxmasterz.
Visually Neverwinter Nights isn't too impressive anymore. For it's time it wasn't bad, especially since it was coming on the heels of epic 2d RPGs like Baldur's Gate II, and Icewind Dale II. I must admit, though; the forest tile set has a certain majesty to it that isn't present in NWN's successor... maybe because NWN 2 doesn't have any real forests. Audio wise NWN is pretty bland. The voice acting is above average, but Jeremy Soule's musical score is so painfully generic that it's anything but memorable.
Neverwinter Nights is a solid title if you're looking to play the modules of other players, or create your own. If you're looking for a good official single player experience look elsewhere. Honestly, those who criticize the NWN 2 OC obviously never played NWN 1's painfully tedious campaign.