Weighted sleep sacks?

Has anyone used weighted sleep sacks of any kind with their babies?? I am thinking of trying something like this (see photo) when both my twins are at the medium size, probably in a few weeks. My boys are almost 4 months, but adjusted age only about 2 months. They are 10 + 13 pounds.

One of the twins is a good sleeper and likes being swaddled, but soon will have to have his arms out. Not sure how he'll do with the transition. The other is not a great sleeper and doesn't get swaddled anymore, but I think he could benefit from some weight.

These things are not cheap though ($30+ each and I have twins), so I'm hesitant! Any recommendations or thoughts, positive or negative?