I'm with most people here. The risks just outweigh the "fun" they can have. Their toes are so small, getting them caught in the slits or the holes can, and in a lot of cases WILL cause broken toes, torn nails, and in extreme cases, serious injury.
The woman who owned Zelda before us used to let her run around in a hamster ball, and gave it to us with Zelda. I took someone's advice, and now use it as a hanging hammock type toy. Hang it from the roof of the cage, and lay fleece in the bottom, fill with toys.