I know that Gary, Garth, and my old Spin Class instructor (don't laugh!) all love the headset mics, but I just never could adjust to loosing my "working the mic" technique that I've become so accustomed to. The vocal dynamics are greatly changed, and unless you sing at a very steady level, you'll miss some of the dynamic changes that can only come from changing your proximity. Then, there's the whole coughing scenario ....
Just my 2 cents.