Very optimistic. I tested it with a subdomain and a domain redirected to the subdomain and......two completely different results. (yes, it has a permanent 301 redirect).
The tool is only accurate if it scours the website you enter for the leading keywords and then checks CPC or CPM for those keywords with Google AdWords. If it is just using traffic ratings then it's only considering half of the equation. I don't think it is checking the value of keywords given its results and you can set your own CPM. If only Google will provide an API with access to the keyword bid data. Nice try though.