These shower rings are great. The balls help them move easily along the shower rod and keep them from getting caught up. They do an excellent job of separating the curtain and liner (which my cat loves, since she likes to walk along the edge of the tub). 

The only thing to be wary of is that they do NOT work with every shower curtain. If your curtain has metal rings around the holes which are smaller than the balls at the end of these rings, it won't work.