PC Magazine reviews have tended to favor the following two (2) products:
August 2004
Spy Sweeper (freeware without live update option or $30 and yearly subscription renewal fees)
Spybot (freeware)
Having said that, read their introductory comments (March 2004):
"Let's be clear: None of the applications we tested for this roundup hit the ball out of the park in terms of detecting and removing the adware, Trojan horses, key loggers, and hosts of other assorted nasties that make up the unpleasant category of applications known as spyware. They're not yet as good at their jobs as antivirus programs are, but they're nearly as important to have on your PC."
Many people on this board tend to favour freeware options such as SpyBot and AdAware. Many of us also tend to run at least two (2) of these tools if not even a third. Each tool seems to have a special niche that they excel in (cookies, registry, and completeness of file removal). This leaves no confidence that one tool is clearly the 'one' to go with. Most people would not be willing to pay good money for a tool that could miss any one substantive area of concern.
Furthermore, most of the popular tools are currently less convenient in terms of corporate rollout and update maintenance logistics, and execution in command line parameters. I would gladly pay for a tools that work as smooth as some of the Anti-Virus auto-update features. This is where Spy Sweeper appears to stands out.