One of my co-workers bought one about two years ago. Every time I saw him at the range, he did nothing but fiddle with different springs and different ammo. He used to buy 50-round boxes of every type of .22LR ammo he could lay hands on.
Probably saved him a lot of money on ammo overall, though.
I was going through a 550-pack about once a month. I never saw him get through a magazine worth. He probably has a dozen boxes, each with 40-45 rounds remaining.
I haven't seen him at the range recently. Not sure if he got it working and tucked it away, or if he gave up. But if I had to bet one way or the other, I know which way my money would go...