Cock rings are great. It's all about proper fit though which can be tricky. A guy also needs to figure out if he likes a ring that only goes around the base of his penis, or one that goes around the penis and testicles. There are sizing guides online, which is what I suggest going by. It's best to start with cheaper rubber rings. Start with the diameter you think you want to try and go from there, either up or down in diameter in 1/4" increments until you get a fit you like.
I personally like a ring that gives me about 1/2" more temporary girth when erect. I also prefer a shaft only ring. At my shaft girth of 6" erect, that translates to a 1.5" diameter ring. The ring should come off semi easily after sex, possibly with a little lube, or soap required. Of course, when flaccid, it should come off easily.
Again, start by experimenting with the cheap rubber rings for a while until you find your size. Later on, you can upgrade to the nicer rigid rings. I like a steel ring made by "California Exotics" their "Alchemy" brand ring. It's a seamless ring which is better for comfort. This ring was $18 after shipping. There are more pricey rings of course, but there's no need to get too fancy in my opinion.
One more bit of advice. Take your time with figuring out your proper fit. If a ring gets stuck, don't panic! If you got it on, it will come off. You may need to "walk it off", wait for your erection to subside and like I said above, use some lube to ease removal. Getting a ring truly stuck, should never happen though unless you are stupid and do something like pump with it on causing too much swelling and/or fluid build up.
"It's all about that fit, bout that fit, no trouble!"
Just sharing some experience!
if not ejacualated.