I personally find them useful at work, where there is a pop machine in our lounge. We have these lovely lazy boy chairs and this engine of a pop machine. These come in real handy. Also here at home, it really cuts down on all the noise around you. No it doesn't eliminate 100% of the noise, I don't think it's supposed to, but the difference is night and day. These also don't bleed out any sound.
I don't see myself walking around or going for a bike ride with these, but in general I'm extremely happy with what I use them for. To each their own though.
"You think that's air you're breathing?"