It can delay ejaculation and for the gentlemen who have a hard time getting hard, or getting and staying hard, this is a great tool for them to get the desired effects. Just make sure you get one that's not too small, lol, or too big.

Big fan

Cock rings are a great addition to the bedroom.  It makes you rock hard and shoot harder as well.  My advice for any man thinking of trying one out would be to try it solo first.  You can get a feel how your body reacts to having one on.

I've been using a sort of elastic band for years.  Never got a real cock ring.  From personal experience, i dont think it delays orgasm.  In fact as another pointed out, it intensifies feelings, so it may make you shoot faster.  I've used the bands for so long I can't tell anymore.  

The main reason i use the bands is because I hate when one of my balls draws up into my body or gets pushed up there by too much pressure (thigh or tongue being the usual culprits) .  Bothers the heck out me cause it just feels so strange.  I know it sounds weird, but i've had friends say the ball thing has happened to them and they didn't like it either.  Sorry if TMI, but just wanted to point out potential other benefit.