I have experimented with different types of 'cock rings' in my private life, and can confirm that the willing subjects always got very aroused, but sometimes it can lead to a premature and unexpected 'finish'
They do nothing for me personally (I don't like vibrating toys, or any other toys for that matter), but men seem to enjoy the sensation a lot.
The best one I've tried so far is the Durex one called 'little devil', because it's nice and soft and comfortable to wear.
However, I would not use them with my clients, because on two separate occasions the toy seemed to have caused the condom to break (might have been a coincidence, but still...), and I'd be too worried about it.
Edit: just realised the OP was asking about the non-vibrating rings, I haven't tried those yet...