Hi Michael,
It is a little smarter than C-<Ret> in slime-repl, handles comments, reformats trailing right parens into one line, and DWIM when a region is active.
Is the DWIM part mostly for use with (the blasphemous :-)) transient-mark-mode? In my setup (which I think is the default) the mark is pretty much always active, so this command generally operates on the region.
Should we conditionalize the narrow-to-region call on transient-mark-mode?
Cheers, Luke