A User of Custom Count and Sum
I installed this add-on for the SUMALLSHEETS function, which I hoped would work like Excel but sadly it doesn't auto-update when one of the computed values changes. To update the sum the formula must be edited or, in the case of several SUMALLSHEETS being used on the same page, the page must be reloaded. I don't see how this is any better than using a longer formula like Sheet1!A1+Sheet2!A1+Sheet3!A1.
I haven't tried any of the count-by-color, etc. functions of this add-on so I can't comment on those.