I like that it scans my site as soon as i install it, so there are no problems in the future if you have not yet installed any plugins or themes with security issues (like XSS).
It's also very easy to use - just click scan button after installation and wait until scanning finishes. Sometimes when loading pages from different locations, it takes some time before page loads completely - but this isn't something which bothers me much since everything else works fine :) Definitely try out WPHC first, then decide whether its worth paying $9/month for premium version - otherwise you'll be wasting money without getting anything more than free trial. My main goal was finding vulnerabilities within theme itself - nothing majorly bad about plugin, only minor things were found.