Sion was okay. He was destined to always play second fiddle to Nihilus or Traya, because he just doesn't have the character to be the Sith Lord master. He's not a bad character, really, and certainly a lot less cheesy than Malak, but he still can't hold a candle next to Nihilus or Traya, who are both far more interesting and potent characters.
Next to them, Sion is little more than a mindless brute, a zombie, the blunt weapon that must be wielded or at least guided by someone with greater insight or a more strategic comprehension.
What G0T0 says of Malak is almost just as true for Sion: "Malak demonstrated no concern for the future of the Republic in his attacks. His stratagems were painfully obvious, intending to crush all resistance, everywhere. There was little thought beyond the complete destruction of anything that opposed him."
Sion is not quite as cheesy or silly as Malak, but he's still the least of the sith lords in TSL.