I use only Plantronics headsets and an Avaya HIC-1 headset cord. I use mostly H-51N headsets and the sound quality is very good. I have had a few complaints about the volume being a little low in a very loud environment, but other than that, I have have very good luck.
There are lots of suppliers out there that sell them refurbished with a 1 year warranty for around the $40 price tag, but I am paying a little less than that.
gblucas