the key to cock rings is getting the right fit. everyone is different so i would recommend trying various sizes and elasticity until you find one that can maintain your erection as well as prolonged wearing without pain.
if you can do what i said above, they work pretty well. i will break mine out every so often if my wife wants a long session. i can last pretty long without it but with the cock ring, i can go twice as long and stay just as hard. it basically keeps the blood in place and keeps the penis hard. again, find the right fit and you'll love it!
many men use it to stay harder longer and even after they cum so that they can continue to fuck away.