What's funny and a little disconcerting is that when you crank the volume, the earpads literally vibrate on your cheek, especially if you're listening to bass-heavy music. It kind of tickles. Thus, it's not a good idea to crank the volume. Rather, these sound best at more moderate volume levels.
The other issue is that they do leak a lot of sound. That's another reason not to crank the volume.
The AfterShokz Sportz M2 headphones aren't for folks who are sticklers for good sound quality. Rather, they're for someone who's looking for a very "open" design that allows you to hear the external world as you're listening to sound through the headphones. When I was on the treadmill at the gym, I liked them for listening to the TV broadcast; I thought they were excellent for that. I also liked them for making cell phone calls. In other words, they were better for speech than music -- they'd be good for audiobooks and podcasts as well -- which is not surprising considering bone-conduction technology was originally designed for military communications, not music-listening.
So while these ultimately aren't for everyone, they will appeal to people who want comfortable headphones that you don't have to jam in your ears (or wear in your ears at all). Just don't expect them to provide a great listening experience -- for music, anyway.