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After reading the reviews for these on Cnet, I thought I would try them out, figuring they would sound pretty good, and $40 seemed like a reasonable deal.
When I got them for Christmas, I was blown away. Nothing else even compares to how great the PortaPros sound, it was like discovering all of my music for the first time! Even though these headphones were originally made in 1984, they still compete with $200 - $300 headphones in terms of sound.
However, since the design is a bit old, they don't have any noise-cancellation capabilities. While this isn't ideal, turning up the volume on my mp3 player did a fine job of blocking outside noises. Similarly, if you are in a quiet room, it should be pretty easy for nearby people to hear a bit of your music, but this isn't a problem in louder places.
One more minor complaint I have with these is that they aren't the most comfortable headphones around. Don't get me wrong, they are very nice to wear in most situations, and you shouldn't have much of a problem with them. However, many times I've gotten my hair caught in the sliders or hinges, but I've gotten used to it, and it's not too terrible. One more thing is that after a half-hour or so of sleeping on them, they can't hurt the side of your head a bit if you aren't careful. However, I think that's simply something that comes with all over-the-ear headphones.
Also, these are very portable. They have a unique folding mechanism that makes them easy to carry in any bag, or in your hand. Plus, it comes with a free carrying bag!

All in all, these are a huge bargain, and I would recommend them to anyone wanting excellent but cheap headphones.