It's good for grip strength, fist strength, wrist strength, and forearms. That's why boxers do it.
For non-boxers, I don't see much value.
It's a trendy exercise right now. I think it's slightly over-ratted.
But, that said, we all need variety in our workouts. If you are bored with your workouts and you need a fresh set of exercises. Then, by all means, sledgehammer the fuck out of yourself.
Sometimes we just need something new to inspire us.
If sledgehammering is the exercise that will get you off your ass; then go sledge fucking crazy.
I think G just needs a new routine with some primal, back woods, gorilla type shit. Use a heavy hammer, use your legs and entire body to generate power, and explode. Beat the fuck out of something!
Breaking stuff is fun and therapeutic.
These guys are using bad technique. They are not using their bodies to generate power and they are not exploding. You gotta put that hammer through the fucking floor!
This is how to do it right!
(Fast forward to the 1:00 minute mark)(Observe the good form around 1:30)