I had a Restrap one and although I used it for a while and it worked well enough, I never really got on with it. For some reason, it always felt awkward to use. For example, although it has pockets on both sides, as soon as you put something in one side, the other side becomes almost unusable.
I replaced it with an Apidura one which seems to work much better – not much bigger on the outside, but I can always fit more stuff in if I need to, and the pocket arrangement works better, with more attention to detail, like the provision of a hole for a charger cable, zips that are easier to handle with frozen fingers, more waterproof, a little zip section for phone/cards, etc.