Perfect size for loose curls
The Cloud Nine system is excellent in that it's modular so you can choose how many of each size roller you want (as opposed to most sets where you generally don't get enough of the sizes you really want). The velcro is perfect for those with fine, silky hair who may have trouble with rolling onto hot smooth ceramic rollers.
These 40mm rollers get you gorgeous bouncy loose curls / waves with a similar finish to a salon-style blow wave with a medium barrel round brush. I own all the sizes and these are the ones I reach for the most.
Definitely recommended to expand your Ultimate Set or build your own set.