Considering how I have seen much faster downloads then that site provided, I am going to say that for the Northeast US it isn't very accurate. I have downloaded at up to 1.5Mb/second, but the site has to be able to push that much data, and not be bogged down with tons of traffic.
I might try it again later, but doubt it. The only time I have seen anything faster has been at work where we have 3 T1 lines to the outside. I downloaded a file one day where pretty much everyone else was gone for the day and it went into negative numbers (after passing 2.5MB/second). It went too fast for the browser to understand (hence the negative number).