Read this article about speedtest.net and speedtesters in general:
http://www.thinkbroadband.com/news/i/3779.html
Sounds like speedtest.net is one of only two worth using then.
For me, the accuracy of Speedtest depends on which server I use. It picks out Duesseldorf as being the closest server to me, but on this Uni connection it never shows over 6Mbps. When I choose Frankfurt I get the full ~25Mbps.