I can't believe the number of people that say they have played it and finished it.
There are new modules coming out everyday, some excellent online games organised by
www.neverwinterconnections.com and then the whole toolset to build and run your own games online.
The single player campaign is probably the least interesting aspect of NWN. In fact so much so I've ground to a halt in the single player game just before the end of Chapter 2, I'm now using this game for online play and building my own module.
I would say "Yes for me it has replaced PnP D&D at least for the moment." as I don't currently have a pen and paper game of D&D on going (our group is playing Flashing Blades instead).
I'm currently in a regular NWN campaign every Sunday run by Lazybones, and when I get cable I will start hosting my own NWN campaign. So I'll be having 2 NWN games to one PnP RPG.
I think the problem some people are having with NWN is they are treating it like any other Computer RPG. They aren't trying to do anything new. There play style is like they would play Half-Life or Diablo online, IE: They switch on there computer and try and find a game online.
NWN shouldn't be played like that. You want to sign up for Neverwinter Connections, and find/host a scheduled game. The fact you might schedule a game for a few nights ahead tends to reduce the number of 'Hunt/Kill/Level/Repeat' players.
Also so module writers are doing some really clever stuff with the engine like the HardCore Ruleset, bleeding rules, no respawns, no XP for monster kills (just objectives reached), etc, etc. DM'ed games (especially with multiple DMs) can be really interesting with properly interactive NPC's.
I would suggest people that have just played the single player game take a second look.