The NWN OC is dull, very slow and just uninspired. It feels like a demo for the toolset which would have been fine if it was just one chapter long but as it is it overstays its welcome.
I guess if you play it, play it on hard. I remember on regular there was no challenge at all which made the game more of a chore.
Shadows Of Unrentide was an improvement. Still a bit on the dull side but had a nicer variety of locales than the first and short enough to offer a decent play through if you can stick through it. Nothing special though.
Hordes Of The Underdark was the best of the bunch - fast paced, challenging battles and just more fun to play overall. THIS is how the OC should have been done.
So just like with NWN2's Mask Of The Betrayer, NWN is worth getting just for HoTU (though NWN2's OC was much better than NWN's - but still nothing great).