5 Stars given because it's clearly a work in progress, but for an early version it shows an awful lot of promise and potential..
I love the treadmill mode, but agree that an option to hide the treadmill image would be great.. or perhaps a couple of options, one to hide it and another to have just a couple of ghosted rails at the sides for reference and safety whilst not getting in the way of the scenery as much. The auto turning in treadmill mode is a little too sudden as well and the experience would be a lot better if the turning was smoother and more gradual, as though you were actually just slowly turning to follow the route. This is probably the biggest issue with it for me at the moment.
A couple of other minor gripes.. after running around 2km or so the music finishes (or it did for me anyway). Ideally it would be great if there was more music and it went on for longer, as it's actually really good (in my opinion), but if at least once it finishes it could just start again, rather than remaining silent, that would be good.
I noticed that the runner that you're following went through the middle of a tree on at least one occasion which breaks the immersiveness of the experience a little. I'm not sure if the path they follow is slightly randomised or completely set, but maybe going through tree trunks could be avoided somehow anyway. :)
I can't wait to see where you take this with future developments. There is the basis of something I'd use a lot and certainly pay to buy.
Things that I think would make it awesome in the future include:
More maps in different settings & environments
Abililty to use your own custom music tracks (I see this is listed for v 3.0)
Selectable running buddies - male/female etc.