Update 20/2017:
Used this ring for the past several years. Color has held up well through several moves and yearly usage. The edges show signs of wear, especially the inside that rubs against the silver inside ring. It's been wonderful as a fidget ring.

_____
As I'm typing this I'm playing with the ring on my index finger.  It's so fun!  For a few months, after I saw my cousin buy spinner rings, I really wanted one.  This month I graduated and wanted to buy a ring as a celebration for an accomplishment of my life's work.  I'm glad this ring is my first ring I am enjoying to mark this occasion.

Ring is designed so the inner steel ring fits inside the rim of the outer black ring. This allows the outer ring to spin over the inner, as well as rotate, as the picture shows. It's a great party topic to bring up with friends and family.

For ring sizes, my asian hand can fit a size 8 snuggly on my index finger and also fit loosely on my ring finger.

Spinning the rings perpendicular to each other, as Gawin said, is not as smooth as I imagined. The inner ring will catch slightly and can be pushed back in for continuous playing.

The first few moments I was getting it off on my index finger I pulled the black ring too hard and it popped off the silver ring.  They are not loose enough to disengage without considerable force, so there is no fear of the black ring slipping off and getting lost. The black ring can easily slip onto my thumb. Being able to take off the black ring gives the added benefit of having two rings!

Overall this ring has been a wonderful way for me. Owning this ring and playing with it has given me joy and satisfaction. Best luck at deciding on rings, and hope you choose this one.