Ok so I'm not a tele-marketer but I do happen to own a pair of these. A relative of mine bought them from eBay for £75 because they are really into the whole 'internet phone-call' fad. I believe that they were using Skype to make calls to people and this was their primary purpose. However they were passed onto me when said relative moved abroad.
I have been using these for about 3 weeks now and they are pretty good. I wanted to use them for online gaming but because they only have one ear-piece they aren't really suitable, as they do not immerse you in the game-play enough. So instead they have sat in my drawer waiting for the rare occasions that I make voice-calls.
I have used these about 4 times while making voice calls on Windows Live Messenger and they are brilliant. The person I was talking to had absolutely no problems hearing me and said the clarity of my voice was very clear. As a test I even played music aloud in the background noise and my friend said he could not hear any background noise at all. I also used these on Ventrilo a few times and they seemed to work ok on there too.
The ear-piece is very comfortable and they do seem to have a very crisp sound quality, however these is a lack of bass. Everything that comes out of that speaker is so very tinny, another reason they would not be good for anything other than tele-communicating ; Which is what they were designed for so I really can't complain.
Avaya isn't really a well-known make/brand so if I was buying these I'd be a bit hesitant but I can assure you they are quite good and they do exactly what the manufacturer says. They are indeed very light, they have very good sound quality as mentioned above and the design is pretty modern and stylish.
So to sum up, if you make a lot of phone calls via your computer then by all means go ahead and buy these if you have the money. They are well worth it if what you want is voice clarity. Oh and also, if you are a telemarketer. Do not buy these! Do not buy any microphone or communications device what so ever and get a career change. I dislike tele-marketers so much.