When I was exclusively a knitter but hadn't started spinning at this time, I loved superwash merino. I knew I should always treat my knitted items gently when washing, but I liked the fact that I could wash superwash items in the washing machine. When I started spinning and then became an indie dyer, I loved superwash wools because of how well they absorbed professional acid dyes. You can get some amazing bright and vivid colors on superwash wools. In today's market, I'm finding more and more wool breeds being offered as superwash.
But what is it exactly? What process does wool go through to become superwash? Each hair of wool is made up of scales. Felting occurs when these scales bind together. Unless you're purposely felting the wool or the wool breed itself is not prone to felting, you should not wash wool in a machine.
Superwash wool is created in a surprisingly toxic way. There are several different processes that can be used to make superwash wool, but all of them start with its chlorination by caustic chlorine-based chemicals. These chemicals can cause burns and can easily produce deadly chlorine gas.
The hypochlorite often used to chlorinate Superwash wool is similar to the active ingredient in household bleach, but it is more dangerous because it is unstable under the acidic conditions used. The sodium hydroxide (caustic soda or lye) that is normally added to household bleach keeps the pH high so that chlorine gas will not be produced, which makes it safer to use. However, the high pH of sodium hydroxide would destroy wool. Wool is very sensitive to high pH and does best if kept at mildly to moderately acid conditions. This is why we don't normally dye wool in the presence of soda ash, which is widely used to dye cotton and other plant base fibers; although wool takes fiber reactive dyes very well when dyed with soda ash as the auxiliary chemical, the amount of damage can be considerable, if you're not careful.
After the wool is chlorinated, it is washed and the chlorine is neutralized, after which a patented synthetic resin coating is applied, essentially gluing down the scales that would otherwise interact under the mechanical stress of washing to produce felting.
It is surprising that, after all of the chemical changes produced in wool by the superwash process, the wool remains very easy to dye. In my opinion it may not be entirely correct to refer to superwash wool as a natural fiber. After learning what happens to make wool superwash. I stopped purchasing it in bulk. I am guilty of buying the occasional dyed braid if the colors especially irresistible, but I usually don't enjoy spinning it.
I'm not completely anti supewash but I'm not a big fan of it either. I don't purposely buy it, but it's really hard to avoid. Most commercial yarn is superwash now-a-days. Being a wool processor, I prefer to keep wool as is.
With all that being said. I think it's best to be aware of what superwash is, there's nothing wrong with being an informed buyer. It's good to be able to make an educated decision.