The Power Ring is well shaped for the most part. The position of the clitoral stimulator is just right, and makes good contact during penetration. I'm not so sure about what California Exotics calls the "triple contact", the lower portion of the ring which sticks out with the objective of stimulating the receiver's perineum. When I was wearing the ring however, the stretching of the ring causes it to slightly deform and make the "triple contact" angle lower. In this case, the stimulator instead makes contact with the receiver's anus, which could be good or bad, depending on where your partner stands with regards to ass play.
As mentioned previously, the Power Ring is less stretchy than many other "soft" rings made of the same or similar materials. It seems like part of the lack of stretch is due to the thickness of the ring, which is about twice as thick as similar rings. The reason for this additional thickness is to house the two magnets which are molded into the ring. While opinions on the effect of magnets on sexual performance vary, regardless of the outcome, I don't think it's worth the lost flexibility to put the magnets in the ring. The loss of stretch makes the ring a bit harder to put on and take off. It is still workable, albeit less friendly for a first-time user. Though I'm not exactly girthy (4.5" circumference), I found the ring to be a bit too tight, becoming uncomfortable eventually. For larger than average users, this ring may be too constricting. The product page lists the maximum stretch diameter as 5", but it was hard to stretch to just half that size.