I have one but HATE it. It never calibrates the milage correctly. Ended up getting a Garmin. I am going ebay mine (as soon as I find the original packaging).
I tried it tonight, it is pretty sweet! I just wish there was an easier way to skip a song if you wanted too...I am using an ipod touch with it. I haven't even logged on to the site, but I will totally check out the challenges!!