Simply amazing. The gameplay isn't the best in the world, but it is worth it for the Toolset alone.
But this game has ten million times the content to offer than a game like Diablo 2. You can download modules from other players, or better yet, make your own! If you can figure out Windows Paint, you can figure out this tool set. lol. There is a lot of untapped power in this game engine and its tool set.
I must admit, that I was too young when Pen & Paper gaming was at its height in popularity, so I never really had a chance to play AD&D, though I know what it is basically like. So, given that, I understand the slower nature of the game, though I still like hacking my way through enemies in Diablo 2. If you had a hack & slash RPG with a tool set this powerful, that would be a dream come true (Titan Quest, perhaps? One can hope...).
But the true beauty of NWN is that the game doesn't have to be about combat. You can set up mazes, puzzles, NPCs, shops, plots, custom made equipment. You can even fine tune it down to what a monster or NPC has in their inventory to pick up when you kill them.
Bascially, as long as you have the hard drive space, you can make whatever you want, as big as you want.
Throw in the fact that it has multiplayer and you can play online with your friends using your custom made modules, and I think that makes this one of the better RPG's of all time. I know I'm having a blast making my own campaigns.
Definitely worth checking out. They have various packages, such as the Platinum edition, etc. that come with the expansions to it, and they come fairly cheap, so you don't have a reason not to try it! :)
I'm HIGHLY looking forward to Neverwinter Nights 2 because of how good the first one is. You can't go wrong.