100% wool dryer balls are growing in popularity to the point where I have even seen them at my local dollar store, and for good reason. They can save you money in laundry costs.
If you are someone who uses dryer sheets, these wool balls are a great alternative that removes static from your clothes and can decrease drying time by around 30-50%. So, you save money by not having to buy dryer sheets, as well as on electricity. All you do is toss them into the dryer with your clothes!
"Per Toner, “Dryer balls excel in reducing drying time due to their ability to create space between clothes. This separation facilitates better air circulation, leading to faster drying and potential energy savings.” | bobvila
I have had mine for many many years now, and they are still in great shape. When or if they do ever break down to the point of being unable to use, the wool is biodegradable.
Just make sure that what you buy is 100% wool. If you are crafty (and have access to wool) you can also make your own fairly easily.
Here's a link to a consumer review org that actually tested it with standardized loads and measured drying times. They don't work, takes just as long and cost the same in electricity.
https://www.choice.com.au/home-and-living/laundry-and-cleaning/dryers/articles/dryer-ball-review
It's sad that nowadays most "reviews" are just going by feelz and not by numbers. The placebo is strong with this category.