After using the app for a while now it does have issues with the server being down or busy for short amounts of time.
It gets to 98% then stops sometimes although if you take a picture of your mystery plant you can identify it later when you do have connection or the server is up.
It told us the mystery plant on our allotment which we have trouble getting rid of is Jerusalem Artichoke. Seems to do quite well if you get a good shot of a leaf shape. I am impressed by how little it is bothered by a single plant with a backdrop of loads of other plants, it picks out the one I am interested in quite well.
The 'add plant' feature does not seem to do anything, it may be it is waiting for more people to identify the same plant.