These are cheap, and perfect for chucking in your spares box if your mics and stands get a lot of use. The last thing you want is a snapped mic holder and having to embarrassingly wrap it up in gaffa tape. With a few of these in your box, you'll never have to, and at this price, why not get a load?
They hold standard size microphones snugly and reassuringly well, but if you have large diaphragm or more specialist mics, these won't do the job at all. But for standard SM57s of SM58s they are fine.
Ideal as spares, but maybe spend a bit more for your main holders as you do get what you pay for.