Like DK said, adjustablity it great, especially if you're new to cock rings. Plenty of people love, and do just fine with, metal rings, but you certainly have to make sure you get the right size and know what you're doing with those.
Your cock needs oxygenated blood to be healthy. Therefore, it's a really bad idea to leave a ring (or anything that restricts blood flow to any part of the body) on for more than 15-20 minutes; beyond that, and you're risking tissue damage. 10 or so minutes is a better guide if you have any circulatory issues.
Consider removing any hair under and around where the ring goes, too. We've had reports of some very painful experiences with pubes being caught and yanked out in the past here.
Otherwise, this is a pretty popular topic, so I'd suggest checking out:
The threads in this post (scroll down, under "Toys + Lingerie") from The Blank Manual sticky at the top of How To.
These search results. (FYI: You can find just about anything discussed in these forums by replacing 'cockrings' with other search terms. To search stories and the entire site, use 'site:literotica.com search term(s)' instead of 'site:forum.literotica.com search term(s)'.)
just bought my first one, was a nutsnbolts from doc johnson fits pretty good and has little nubs on it to rub against the clit, made out of some kind of rubber.
I have used them in the past but only the flexible rubbery kind. My girl and I especially enjoy them with the built in vibe...she loves it when I'm inside and my whole cock is vibrating!