Pros: They help some guys get and stay hard, the visual is cool, as jewelry I can appreciate that it jsut feels cool. Cons: Chipped teeth, prolonged usage can be uncomfortable.
Personally, I really don't like them on me or others. They pinch, and far from enhancing an erection, they inhibit it -- specifically the metal and rubber rings, and the leather snap straps. That said, the only kind that I was able to wear that actually felt erotic, but looked like something out of the Power Puff Girls' lunchbox, was an irridescent jelly-textured ring that appeared to be more like a wreath than a ring. OK, I have no recall of where I got it or why I had it on LOL
However, there was a pool party last summer at which the hosts went around and personally strapped up guys with the day-glow necklaces underneath their swim suits. The joke was how many times the necklace needed to be looped in order to fit.
Didn't like the day-glo necklaces either. It fit OK, but... day-glo?? Too circuit party.