I did try one a long time ago and it seemed to work. They aren't all the same thing however, some contain minerals, others detergent and some both. And of these types there are further variations.

However, Good Housekeeping tested out a number of different washing balls and found none of them to work better than just using water. Also, the claims made by the companies who make them, and the list of contents concern me. Many claim to contain scientific sounding mineral or compounds that are not real names for anything. The explanation of how they affect the water is dubious too. Things like "energised" are vague and mean nothing in a scientific context.

Some claim to contain concentrated detergent, which is the only ingredient I can see that would do anything useful to your washing. The mineral or magnetic products are unlikely to do anything however. There's little established scientific basis in their claims, many of them list ingredients that simply don't exist. Of course they could be using alternative names for something real, but why use a made up name for something that already has a name people understand.

Laundry detergent remains in your clothes and the machine after the wash. About once a month, I run the wash with no detergent and dead tennis balls to help agitate. I'm convinced I get just as far with dead tennis balls as my brother does with his fancy washing balls.