I’ve had one for a couple of years now and use it most of the time instead of a back pack. I don’t notice it moving about and holds all the stuff I need in a couple of largish pockets.
It also keeps the weight low down on the back and obviously no pull from shoulder straps. Bladder only 1.5l so obviously no good for all-day rides with no refuelling stops. Only other niggle is the tube is slightly short to comfortably drink on the move. I am 6′ 4″ though so might be okay if you are a bit shorter. No problem to drink at stops though.
I also have a couple of “normal” back packs but would buy another one of these in preference if this one goes.