Quick take
They are worth it when a validated shower-specific filter addresses chlorine or another documented concern at shower flow. They are not a true solution for hard-water softening.
When a shower filter makes sense
A renter or homeowner who only wants shower-specific chlorine reduction can avoid the cost of whole-house treatment.
Evidence matters
NSF/ANSI 177 is the relevant shower-filter standard for free-chlorine reduction. Weddell also publishes additional shower-specific test data.
Hard water limitation
Most shower filters do not remove enough calcium and magnesium to produce true soft water. Salt-free “conditioning” claims should not be confused with ion-exchange softening.