I see them all the time around here (there's a Microcenter nearby.)
They're usually pretty slow. I've seen a few that had lasted a while, and a few that died almost out of the box.
I'd guess it's like anything else mass produced by the lowest bidder. Quantity has a quality all its own or somesuch. If they're cheap, go for it.
I ran a few thumb drives through the laundry, so I try to think of them as reusable but disposable. Like Floppies.