From what I read, it's one of very few that are actually trustworthy ... many try/buy ones will exaggerate a relatively minor issue (eg. cookies) and then extort payment to clean it - some are even spyware themselves!
Many also leech off the same work that goes into the free ones.
http://www.spywarewarrior.com/rogue_anti-spyware.htm
Check here before you buy, as it's better to donate for Spybot S&D or buy Adaware pro, than to support the leechers.
PS. Not sure I agree with their positive rating of "Pest Patrol", since it has severe tendencies to paranoia, tagging many things that WOULD be inappropriate in a corporate environment, as although their effects and actions are only as chosen by the user, they would be against the wishes of a system administrator.