Others say...

"Great for Internet Phone, travel, MacBook Pro"
These fold nicely for traveling. I have been using this headset with my MacBook Pro overseas, and using Gizmo internet phone (like Skype) and they have been working great. Just plug the USB adapter into the notebook, and select the headset under System Preferences - Sound (for both input and output) on the Mac (OSX 10.5 Leopard), and they work.
Sound quality is very good, no echo with a good internet connection. The noise canceling microphone works well. They fit and are comfortable. They adjust for large or small heads - via the typical sliding head band.

"Very clear and great noice canceling feature."
I am extremely satisfiled with this headset. I had used many others till now but i guess this is the one with a great crystal clear sound. I frequently Skype with clients all over the world and the call quality is so better.

"Great sound, not so good QC and construction"
I ordered one and it came in defective. The USB adapter was bent and can't be recognized by the PC. I then tried the direct mic/headphone line-in and found out the mic didn't work. The sound quality from the headphones were very good, though.
This is my first time using Amazon's return. It was very easy to ship the defective one back. The new unit arrived in 2 days. No hassle at all.
The new one works well. The mic's sound quality is pretty good, too. However, this product's construction is not sturdy. The folding structure feels flimsy and the sliding adjustment looks if it will fall apart very soon! It also pulls off my hair, too. I often casually toss my $20 Logitech around (on sofa or bed or carpet, that is) and have had no concern of breaking it. I feel this unit needs "Handle with care" warning stickers all over it!
The sound quality is probably 4 or even higher, but the structure and built can only be 2, not counting the questionable QC. That's why I am giving it a 3, regrettably.
One remaining question is that the shipped unit is not exactly the one in the picture. The one I received does not have that good sized in-line DSP module. I do not know if the mechanical structure between the two versions are identical.

"Incorrect Item"
This item is not what is listed. While the site says DSP400, the headset is actually the one shown here: Plantronics Foldable USB Stereo Headset (Optimized for Laptop Use) (Audio 470 USB). I'm unsure why the box says DSP400 when the .Audio 470 is exactly what the headset is. The microphone works and sounds great, and the headset is just as comfortable as the real DSP400, but instead of being a true USB headset, it has a USB sound-card that you plug the microphone and headphones into. When the volume on the headset is turned all the way up, there is a low buzzing noise that can be heard, which was not present in the original DSP400. Instead of the digital volume control with digital mute button (my favourite part of the original headset, since it controlled default computer volume and not just the headset) there is an analog dial and a slide switch for mute.

I'm not sure why I was sold a fake DSP400 when the .Audio 470 is the same product, but $40 cheaper. I'm disappointed that the one place I found that was advertising the original DSP-400 turned out to be false. I will be returning the headset and investigating what happened.

"Be aware - New version is different from shown, no PerSono"
I just received my headset from Amazon. The latest version from Plantronics is called the .Audio DSP 400 and is different from what is shown in the Amazon picture. The description also talks about the PerSono equalizer software that Amazon says can be downloaded from Plantronics (this was one reason I chose this headset). The PerSono software has beeen DISCONTINUED by Plantronics and is not compatible with what is now branded as the .Audio DSP 400 headset. There is no replacement software. I haven't used the headset yet so no real perspective on performance.