i bought the 2 for 20.00 pack of these a few years ago. They work ok and are decently accurate. Ive used them for rehearsal wo problem but dont know about a gig. They sometimes fall off the headstocks. I need to be able to mute in between songs when i tune on a gig so i dont think a headstock tuner is practical but ymmv.I just got a pair of these D'addario clip ons and i have to say - they are tiny
Here's are some photos - one on a Fender headstock and the other on a Gibson for ref.
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No idea how they will perform in a live setting, but i'm building my next mini board w/ no tuner, and will see how it goes.