I've used it and it is pretty mediocre or at times just inaccurate. I get so many false alerts from Google about the site being down that I've turned it off.
As another comment puts it, it's just the wrong tool for the job.
I've used a modified version of the script for over a year now and was able to determine unreliable sites with it. Of course there can be temporary server issues and when you try to access the site it works again.
The script I wrote checks multiple sites in one go and sends one mail per script execution, if there are errors. I found that more manageable, the code for it is on GitHub
https://github.com/yaph/sitemonitor
I just set this up. Works great! I recommend you run it once from the Run menu to make sure you entered a domain name correctly. You can comment out this line to get an email every time it runs (good for testing):
As another comment puts it, it's just the wrong tool for the job.