I believe that the people who sell it subscribe to this list.
People are reluctant to say what they actually use on Internet lists.
Personally, I use everything I can get my hands on. I also took a look at Pingware from Bellcore, which has a different design philisophy. Whereas ISS very efficiently probes for security holes and will tell you what they are, Pingware is an automatic hacker and highly effective. If your company can afford it, get both ISS and Pingware.