(on a side note, am I the only one annoyed by the fact that Nihilus' name is never said in the game?)
If you ask Atris, after you defeat her in the Holocron room, "Who is Kreia?" she actually mention Nihlius by name, if I recall correctly.
Sion...really didn't live up to expections. He was nothing more than a heir to Malak's legacy, of brutality and destruction. His ability to resurrect was overrated, and you could kill him in 3 tries without breaking his will. When he finally gives up and when you break his will, that was okay...as well as if you are able to romance with Sion. Still, I would have liked it more if Darth Sion broke up to tiny little pieces after being killed off.
Sion is, just like Malak, nothing. They had caused lots of damage, they were responsible for death and destruction...but they were weak on the inside and powerless. Malak was secretly afraid of being overshadowed by Revan...and Sion's only reason for living was to serve his master, Darth Traya.