Do a search on "cock rings" in this forum and you'll find out what they are used for and what people think about them.
As for metal cock rings, I'd stay away from them, unless you can find one that fits perfectly. THey usually only come in 3 sizes (small, medium, large). For me, the medium is just a bit too small and if I get a full erection, it hurts. But the large is just way too big for me. If you go with the rubber cock rings, they give a bit so even though the medium is a tad too small, it stretches and feels fine.