Any rotary sharpening thing--I've bought a couple of things. None have worked well.
I am not a gadget person,--much rather collect fabric--so I don't have a lot of gadgets I have bought that I've not used. I did buy a bias binder maker once and haven't used it yet, but I may, so I don't consider it a waste...I'm glad I have it....it wasn't really very pricey or big, so didn't take much room in the suitcase.