The problem with playing darkside in KOTOR was that you ended up having to play an antisocial jerk who lived to be hated by everyone he/she encountered. As such, I didn't find playing darkside to be very palatable. I'd much rather have played darkside as a scheming, manipulative SOB who got everyone to just love him for his own selfish ends -- kind of like a Darth Bill Clinton. To me, that would be much more "realistic" for someone who is looking to become a powerful Sith leader.
That said, KOTOR is a remarkable game. It's not open ended, which some people think is a bad thing, but it's probably the best realization of the old idea of an "interactive novel" or "interactive movie" I have seen. It is very immersive, and although you always eventually get to the same point, lightside or dark (with minor ending variations), it's an oddly compelling game, with some of the best voice acting and NPC subplots I've seen. The redemption of Yuthura subquest for instance (if you manage to work things so that you meet her back on Dantooine) is very, very well written and in its own way quite moving. So is Jolee Bindo's story.
It's one of those games that, for what it is, got everything just about perfect. Some people want it to be something it's not, but that's a different problem. Unfortunately, LucasArts has made sure that Bioware made the thing almost impossible to mod, but there are some dedicated hackers who are working hard to crack that. I think all they have left to crack is the modelling, which is the hardest.
Excellent game. I played it the second time not too long ago. I managed to do all of the things right that I should have done the first time. I managed to complete many of the subquests, and that made for a much better story and a lot more action. Great game. I would have liked to be able to max out on force abilities on both light and dark, but that might take the fun out.