Usage
The text "WTF?" is a link to Help:Edit summary, while "ALWAYS" is a link to https://xtools.wmflabs.org/editsummary/en.wikipedia.org/YourUserName, which displays the percentage of YourUserName's past edits that included a summary (as well as percentages over time and by edit type).
YourUserName in the URL is derived from {{ROOTPAGENAME}}, so if you need to use the box outside of your /User:YourUserName page hierarchy (e.g. on some discussion page), you can supply an unnamed parameter with your username explicitly specified, as in:
{{User edit summary|Jimbo Wales}}
which produces:
There is also an optional left-text parameter, which you can use to specify text other than the default "WTF?" for the left side of the userbox, e.g. your edit summary percentage at last check:
{{User edit summary|Jimbo Wales|left-text=100%}}
That produces:
Finally, there is an optional frequency parameter, which you can use to specify text other than the default "ALWAYS" in the right side of the userbox. For instance:
{{User edit summary|Jimbo Wales|left-text=97%|frequency=almost always}}
produces:
| 97% | This user almost always leaves an edit summary. |