I tried using this strap for a little while but eventually abandoned the idea of trying to play classical guitar standing up. The strap works in that it allows you to stand, but it's not very stable as the guitar tends to want to lean away from your body, and you have to keep a hold of the neck of the guitar all the time.
After a while the strap becomes a little uncomfortable as it digs into the neck which is trying to support all the weight of the guitar.
So overall, the strap works but personally I gave up on the idea and just went back to sitting with the footrest.