I've had it for a few days now and it's great! If you lock the screen, it will stop tracking you though (I actually JailBroke my iPhone just to install Insomnia because of this app! Now, I just throw it in my pocket and when I get home, email myself the KML file and look at it on Google Earth).
The details it gives are great. Speed does not work properly when going slowly (it keeps jumping between 20MPH and 0 when I'm walking at about 3)but apart from that it's pretty accurate.
Well worth £5.99 if it means that I don't have to carry my 7 year old Garmin with me.