It's a pretty simple app, just touch the "start test" button and run the test to see your download and upload speeds measured in Mbits/sec.
Since using it, I have noticed that LTE speeds depend on your location and thank God, compared to other locations, my home has the fastest speed at 12Mbits/sec download, while at work I get between 6MBits/sec and 8Mbits/sec.
So if you are into this sort of thing, that is your data speeds and the sorts, then get this free "Free Speed test" for your Lumia device, preferably the Lumia 900, since it's the only LTE Nokia windows phone in the market. I mean you can use it on your 710 or 800 but who wants to see data speeds on 3G phones?
You can get Free Speed Test from the marketplace for free!