Once, it had a vibrating thingy on it. She loved it, I personally didn't care for it. Think it was called the "dolphin", or something on that line.
Now, I use the honey nut cheerios, so she can have a healthy heart, while getting her protein for building muscle.
Oh you gents are something else. Lmfao
I do a lot if fetish and they are common there but once someone had his junk get all purple and full of bulging veins so he had to cut the rubber ring off. That would be scary to go through
I've used various stretchy latex vibrating rings (no risk of being too tight) for years with gentlemen. If used correctly, they can be quite a nice novelty. Some feature double vibe spots placed appropriately for her AND his pleasure. There are many types available, so have fun experimenting. You can get them inexpensively through Condom Depot, Condoms.com, and even Amazon.
Now, if you are going with a more rigid ring to help your erection, you do have to be careful and not leave it on too long. I stay away from those for the very risk Lexxxy mentioned.
I did have a lover with a penile piercing who would interchange metal rings and bars. That was pretty interesting, but not what I'd recommend for the faint of heart. The healing process can be challenging, especially in the mornings right after it's done. (Oh, ouch.)