I've had this one for about 4 years, though I haven't used it much because I bought a pair of Gramercy Tools holdfasts that work much better. However, I'm working on mortising the legs for a second portable workbench for the class I'm teaching, and those were out in the van with the other bench.
No problem, I thought, I'll just use this one. It only survived two mortises. Give it a bash, work on the mortise, the stupid thing slips, bash it in again. Then the top just popped off, clean break at the neck.
Cast iron is just too stiff and brittle for holdfasts. Stiff means that the holdfast doesn't hold well. It sets poorly and then slips out easily. Brittle means that it doesn't stand up to the repeated resetting required. A holdfast like this is just annoying to use. It's one of those cheap pieces of junk that reinforces the belief that hand tools are hard to use.
For a proper holdfast, get thee down to the spreading chestnut tree where the village smithy stands. He'll forge you a wrought iron holdfast with the ductility and spring to set well and take a beating.
What, no village smithy? Then the modern equivalent is the Gramercy Tools holdfast. Made from mild steel rod and shaped in an automated wire forming machine, these things work great and will take anything you can dish out (read the patent for the manufacturing process here).
Gramercy Tools holdfast: the right material for the tool.
Get at least two. Might as well order a couple extra, so you can keep a pair in the shop and a pair in the van (I'm sure you can come up with a valid reason to drive around with them). When they arrive, wipe off any machine oil residue, then wrap some sandpaper around the shank and give them a spin to roughen the surface with scratch rings. They'll hold like pit bull velcro.