This is an incredible game ! The most adventure and role playing I've ever had and dreamt about. The game not only lies in the single player missions, which are interesting, not only the regular multiplayer aspect but there's a whole community of developers who made free missions which are maintained by a vibrant community.
Although you have to pay for the game, now a days you will find Neverwinter Nights platinum or diamond under 10$ (6.5€) ; those versions include the expandions Shadows of Undrentide (SoU) and Hordes of the Underdark (HotU) which you'll need for a lot of online servers and will give you many gameplay advantages ! Also with the platinum version, not sure about Diamond, you will get a very nice manual which you should refer to a lot to build your characters and use the many gameplay features to their fullest.
I've spend over 3 years playing this game so I've seen a lot of it and believe me that this amount is puny compared to all the fun you'll get from the game ; at that price, only the single player missions would be more than enough satisfaction but the fun just start with those. This game has a very VERY solid gameplay !