Finally I've found the one headset I've been looking for that can serve as a PC headset and a handset/mobile phone headset. I was pleasantly surprised by the Plantronics MX500i 3-in-1 VoIP Headset. I figure at an MSRP retail price of $60, it had to be good but it was even better than I though. You can actually get it for $38 including shipping.
The MX500i 3-in-1 is a product that has both a 2.5" headphone plug that plugs in to most regular cordless telephones and a USB adapter that plugs in to any computer. I'm happy to report that both operating modes worked superbly and the ear design works well and isn't unwieldy.
I've had plenty of disappointments with USB or BlueTooth headsets which almost always seem to be limited to narrowband applications because they would only support 8 KHz digital sampling. The Polycom Communicato C100S continues to be my favorite USB speaker phone solution ($100) can support 22 KHz digital sampling and it supports wideband applications such as Skype or Counterpath's software-based SIP phones which support G.722 wideband codec (see this article on an explanation of wideband codecs).
Hoping the Plantronics MX500i would at least match the Polycom's capability, I was pleasantly surprised that it even supports up to 48 KHz digital sampling. In fact it was louder and clearer than the Polycom Communicator speakerphone and I would dare say that it sounds about as clean as a studio microphone though it's probably a little more constrained in dynamic range to filter out noise. Here is an uncompressed WAV sample I recorded for you to check the quality.
If I had to give it one criticism, I would say that the USB adapter and headset along with all the cabling combined is a little harder to tidy up and travel with than the Polycom Communicator which tucks everything in to a neat little package. Then again, the Communicator costs more than double the money and its sound quality while the best for a USB speakerphone can't compare to this high quality headset.
So in conclusion, I would say this is an affordable solution that sounds even better than the Polycom Communicator though the Communicator is still useful for hands-free and ear-free operation. If you're looking for something in the $40 range or you're looking for the best call quality, the Plantronics MX500i is for you.