other side of the coin
I think they are great. I have used them for years and still do.
If not set up right or properly maintained then they can we a waste of space.
They have a huge surface area for colonies of aerobic bacteria to set up shop but it realy is all in the way the water is passed over the balls.
If care is not spent on ensuring that the water passing over the balls is well filtered mechanically then they will soon get clogged and get smelly. So always use a really good pre filter or micron bag. I use the latter.
Also you must ensure an even covering of water flows over them, I always use rotating spray bars.
This bioball tower is about five feet tall and has a rotating bar. Also Around the inside of the tower are shelves to stop the water running right down the tower, it pushes the water back onto the balls.