PSwG is one of those 'WTF!!!' animes: not surprising considering it came from the people who brought us FLCL and Magical Shopping Arcade Abenobashi. One of the more unusual aspects is the 'alignment' switch between the angels and the devils: Panty and Stocking are definitely Chaotic in their methods while Skanty and Kneesocks are clearly Lawful to an extreme (they're still good and evil, though, respectivly).
Incidentally, though it's rather obvious that Panty is a slut, Stocking's flaw is she's a glutton. Aside from being just generally evil, Skanty and Kneesocks haven't demonstrated any vices of their own to the point I've gotten in the anime.