Heard of them, you bet. I've got four of them. They're also called "Marimo balls". Around here, they go for about $3US each.
Mine are about 2" in diameter (about the size of a raquet ball). They're very plush, and seem to do well in low light (mine look great, after four months in about 2/3 wpg).
They don't seem to grow very fast, but maybe that's because of my light. I've heard that as they grow, they get hollow on the inside. Then, if you have enough light in your tank, supposedly some of the oxygen they produce gets "trapped" inside the ball and they start to float; i.e. they'll rise a bit, release some gas, and then sink.
Pretty much zero maintenance (sometimes you flip them over if the bottom starts getting pale), and look pretty neat.
edit: also, the pictures can be kind of deceiving. If you're wondering, they're not "slimy" at all. "Furry" would be a better word.