I just checked the purchase date on my Audio-Technica PRO8HEx headset mic and I began using it five years ago. While I owned lots of expensive handhelds, and the audio quality of them was considered somewhat better, the difference was marginal at best to trained ears. However, when it comes to doing live performances, IMO, the headset mic is the best attribute a performing musician could ever imagine.
Pros:
1. Freedom of movement
2. Excellent voice quality
3. No worries about brushing against the mic
4. Ability to maintain eye contact with audience
5. Ease of making keyboard changes on the fly without worrying where the mic is
6. Ability to set up for the next song while you're still singing another
7. When rigged wireless, interaction with the audience
8. No mic stand or boom to carry
9. Lightweight and compact
10.Inespensive (Mine cost me $125)
Cons:
1. Marginal difference in vocal quality
2. I can't think of another....
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)