Great audio, lousy construction and warranty
The audio is great, and I own more than one set of these. However, there is a widespread problem with the plastic headband, and the PSB warranty / customer service is completely deficient. My headphones started to show cracks in the plastic headband after a couple years, but they were cosmetic so I didn't worry about it. One day I sat down in my office, picked up the headphones, and found I couldn't get them to stay on my head. The plastic had completely cracked through, and so the headphones would no longer clamp over my ears. Search the web for "M4U1 cracked plastic" and you'll find this is all too common. I suspect this is a problem only in certain batches of the product but of course I really have no idea. What I do know is that their warranty says that if your headphones are over 3 years old, that's just too bad for you. And if, like me, your headphones are between 2 and 3 years old they'll replace them for half price. I firmly believe this is an issue of faulty material construction, so I do not find that policy to be reasonable.
Pros: Great audio with flat response
Cons: Plastic headband can crack under normal use, rendering the headphones useless. Warranty is inadequate, given that this appears to be a problem with faulty manufacture.