I'm playing th game with the system in my sig, the game has crashed maybe 3 times (once due to task switching). And I'm into chapter 3 with a level 17 char. I find the game to be quite stable. The graphics are great, just not groundbreaking. Sound is pretty damn cool. There are a few tiny annoyances (pathfinding for one) but no major bugs.
Multiplayer is like original Diablo, or FPS games. No centralized servers, but the game has options for enforcing legal chars and class/level restrictions on items. Also, the game has an option for non local chars served only on the server.
The game's huge. MP is a bit slow/laggy, but they have been patching it. I didn't expect to play MP for a while until new modules came out.
My personal advice: Wait for the game to get cheaper and the MP community to develop modules and persistant worlds. Unless you want to make those worlds, or you're hungry for a Bioware RPG. I for one really like NwN compared to BG for the simple fact that you don't have to control 6 different party members.