Even before I played the game the big question was, "Who and what the hell is this guy and what makes him so badass?"
When I went up against him and defeated him, I didn't feel like I had just faced the biggest baddest planet killing mofo out there.
As the game progressed I don't really feel like my questions were answered. I have no idea who Darth Nihilus is. Perhaps that was intentional, but I still have some kind of grudge against him. This is probably because the concept art made this guy look like he would be the person to beat in order to defeat/takeover the sith. My expectations were way too high for Darth Nihilus.