Like many of you, I am disappointed in the way Nihilus turned out... on my original play-through, I was practically crapping my pants when I went to confront him on the bridge. I think it would have been a much better fight if Visas and Mandelore had been thrown across the bridge or incapacitated- the 3 on 1 situation made it far to easy. I think maybe that Visas should have had to struggle more to stay at your side, or (Dark Side) could have sacrificed herself for you. From a purely gameplay perspective the fight was too easy. Then again, most of the KOTOR II fights seemed too easy- it would have been nice if the gameplay difficulty had effected the HP and statistics of your foes. On 'Normal' there were practically no difficult battles, and on 'Difficult', most of the bosses were still fairly easy.
From a purely character standpoint, Nihilus was actually pretty cool. I would have liked it if the battle with him had lived up to his displays of power in the cutscenes and movie, and it would have been cool if when Visas had pealed back his mask, it would have revealed some shocking discovery, but I thought he was a well-concieved character.