I'd hate to be without them. Like any tool their value will vary with what you do. I keep mine handy hanging in a knotted cord in an otherwise useless corner.
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I use one in place of a board jack at the bench.
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Of course, they excel at clamping irregular surfaces. I also use them as small parts holders; any time I paint myself into a corner and have to mill a small blank I clamp it into a hand screw to make it larger. This allows safe router table work holding, sanding, drilling. Anywhere that clamping the small part directly is awkward, clamp it in a hand screw and clamp the hand screw to the machine or table.
I have leather liners on some for delicate work, grooves in others for holding round stock. I think the time Rockler had them on sale for 75% off during one of their garage sales back in the mid-2000's could have had something to do with my collection. The 4" are Harbor Freight I think. They work fine but the larger HF hand screws use an odd thread that slips. Glad I only tried one of them ;-) New Jet and Rockler along with some used Jorgensen and old no-names are the bulk of mine.