Those look cool!
I’ve used a product called sandbells, which are a similar product but without handles. It takes some grip strength to hold a sand-bell, but I like them for that exact reason. I use them more for grip strength and tosses than swings, and they don’t do floor damage like a kettlebell would do.
I also use sandbells and like them a lot but those Dumbags look good to me as well. What I like about the sandbells is the reduced risk of injury. When they drop and fall on your foot, nothing happens (tried this out up to 12 lbs. – don’t have heavier ones). Since I like to play catch with them with my clients, it makes a great tool for those with grip issues or hand/eye coordination. It looks to me that the Dumbags could be employed in the same fashion.