I think you should get it, but only if you plan on getting NWN Gold, which contains the Shadows of Undrentide (the expansion) too. Then pick up Hordes of the Underdark (the other expansion too)
Alternatively get the set which has all 3, I just can't remember the name of it
Basically, the OC isn't the best, a bit boring at time in my opinion, but the expansions are good, and there are some good bridge mods too.
It comes to it's best online though, and you should really try Zirocs server, Escape from Undermountain for hours of fun.