Good in theory, falls short in practice
It's light weight, packs down small in a larger bag, but it left me feeling completely underwhelmed and disappointed.
Even with the foam backing, the bag is still really floppy. When loaded, the foam tends to begin working its way out of the bag which looks sloppy as well as just being annoying.
When partially loaded, the bag lacks the rigidity to hold any shape, and the contents pool up at the bottom in a big glob. This becomes a catch 22, because the bag's straps aren't really padded enough to hold much weight, but the bag definitely feels better on the back when it's stuffed to the brim.
In the end, I found a slightly smaller, heavier hydration-capable bag from Osprey to be a more useful approach. I pack items within this bag, then pack that into the large trekking backpack. Sure I'm sacrificing a bit of weight and space in the large backpack, but I'd rather have a solid platform to make the day hikes enjoyable than be such an ounce-counting nut job that I have to be miserable during the fun parts of my trips because I have a floppy stuff sack for a daypack.