I got to demo one of these 5L powder bags up in revelstoke, BC for a heli day during Flow's Tailgate BC. I was really impressed at how compact and light the 5L was. I always kinda assumed that the ABS bags were super big so I never payed much attention to them or thought about using one until I saw the 5L in real life.
It really doesn't weigh much more than a normal riding pack, and it has the space for avalanche essentials. I was spinning off some decent sized cliffs and pillow lines with it on and it wasn't in the way and didn't throw me off in the air. I had my shovel, probe, water bottle, extra shirt, and a sandwich.
I ended up buying one for hiking/riding bigger faces, and resort riding because I'm at some places prone to large in bounds slides. I just really like the size and weight because I have no excuse not to wear it while riding. I also bought the 15L top that you can swap out with the 5L for longer excursions. I usually wear that one when I go out sledding all day so that I can bring a little bit more gear. I really like how the bag toppers are interchangeable so you aren't stuck with one option once you buy the bag.
The bags are pretty waterproof as far as I've noticed. I left my bag in the snow while I was building a jump for a few hours and nothing seeped through and made my extra layer inside get wet.
The whistle on the chest strap is really nice too. That alone can save your life if you end up separated from your group and stuck in a tree well
my only gripe is that the leg strap buckle was getting snow packed into it when I was heli boarding because I was taking the pack off and putting it back on a bunch of times during heli loads, and the rotor wash was getting snow stuck in there.