The other benefit is that I offer (rather, will offer -- don't have it replicated yet) geographically distributed, redundant checking for up to 10 sites. This is a bit harder (and more expensive) to accomplish for the general developer.
Regarding the method of detection, I actually use a standard "fopen," to the hostname and port you specify, rather than a straight ping. In this way you could test specifically for your webserver (port 80) or SMTP server (25) rather than a generic ping. I even use it to test custom socket server platforms I have running.
Does that answer your questions? Change your mind about the value of the service?
Thanks!