IMHO, the one at Harbor Freight which is made in China and marketed as a stud finder is useless. I forget how much it is, but it was cheap and not worth it. I even use my Garrett Carrot as a stud finder.
But I don't love the Garrett Carrot. I see it as the best house in a really bad neighborhood. I have yet to use a pinpointer that I would endorse, but the Carrot is at least acceptable. It works (more or less).
I would love to see someone (hello ML, XP, Nokta) come out with a really good pinpointer. The world is your oyster, But, perhaps they don't see how lame current pinpointers are, or the problem is harder to solve than it seems.
My pinpointer is definitely the weakest tool in my toolbox, but hope springs eternal.