You have to tie and tighten them so that they won't slide around. It takes a fair bit of fussing around at first until you get it right.
I have a pair. I see a few people on campus walking around in them. One guy wears them 100% of the time, even in winter. I don't know how he does it. Another guy I saw tied his really well so that they almost looked like normal sandals. Mine look like obviously some kind of homemade monstrosity.
The nice thing about them is that the soles are really sturdy. I can't see how you would ever wear them out. The holes you put in them don't wear out and the lacing that goes through the holes stays pretty firmly placed so any sliding around is just a matter of tying them better.
Female, 5'3", 49, Starting weight: 163lbs. Current weight: 135 (more or less).
Starting squat: 45lbs. Current squat: 157.5 x 2