I've always used them, but lots of pros don't.
My fingers change size over any given night so I'm not sure how I'd get on without them.
You've got to change them as soon as feel changes, they do get worn out of course. It's something to consider when you switch from your strike ball to your spare ball. The ball most used will always be more worn than the spare ball, giving you slightly different feel if you don't keep them grips fairly new.
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