As with any large-scale data collection, there will probably be outliers. However, it is the only data-collection service of this nature that I'm aware of. If you can point me to a bigger/better global internet statistics resource with publicly available results, I'll certainly use it.
We (MyBroadband) also have a custom Ookla product:
http://speedtest.mybroadband.co.za/
We use this to get an accurate breakdown of the local internet landscape, and filter any weird outliers that will skew the results (tests from core network nodes, for example). We are waiting on some more data with the current roll-out of Telkom 1Mbps/2Mbps ADSL.