Welcome to Librarium Online!
I haven't used any of the chaos special characters yet as I have not played a game big enough to justify the points for the likes of Archaon. I am sure that if my friends and I play a large game then some of the special characters will have to come out to play. Special characters in general are expensive in points terms and are unlikely to kill their points equivalent but can still be entertaining.
Helper monkey, bring me beer and donuts
Haven't used any special characters yet, thinking about auditioning some of the minor ones like Scyla the spawn or Aekold Helbrass. Some certainly aren't worth the time or the points spent on them (like Galrauch. Powerful character, massively expensive, bad side-effect, crap model...).
"Peace, through superior firepower."