For the price, not so great.
I'm in the lawn service business. I wear ear buds about 8 to 15 hours a day to listen to my mp3s and block out the engine noises (which are pretty loud).
I got these headphones thinking that maybe they would block outer noise a small bit and give great sound considering the great reviews. This was infact not the case by any means. My Jvc earbuds get way better sound quality and ofcourse, block out 90% of outside noise. I didn't expect these headphones to just completely mute outside noise. However, 40% blocked wouldn't be so much to ask for considering the price.