I worked (very part-time) a typesetting job for 10+ years. We were making a journal with (sometimes) pretty narrow columns. Microtype was surprisingly helpful, while remaining (almost) invisible to a layman's eye. (Once you know about it, you can spot the differences in letter widths sometimes.)
Interestingly, Guthenberg used very similar techniques with varying the shapes of letters so that the "blackness" of the page could be more uniform.
My favorite feature of this package is how it expands/contracts letters so that hyphenation is almost entirely gone in a fully justified text. I personally hate the hyphenation of words: they sometimes create ambiguity (incidental hyphens at the end of a line vs hyphens in the original word) and the hyphenation points always seem unintuitive to me.
Fun fact: in Polish typography, you repeat the hyphen at the beginning of the next line in the latter case (hyphenation when there is a hyphen already).
I should warn you that the stretch that the package performs does not preserve the vertical stem widths. Therefore, especially at small point sizes, there are perceptible weight changes.
In my opinion, the default font expansion value stretch=20 is not conservative enough. I would personally recommend not only reducing it but also using the selected argument to further reduce the expansion of glyphs whose geometry distortion may be more easily perceived.
Came here to say the same thing. Also, it pushes commas, full stops, and hyphens to hang over the margin. The typeset outcome is much more pleasant to the eye.
It's hard (impossible?) to justify narrow columns in a fashion which is pleasing to the eye if you don't hyphenate, unless you employ glyph squeezing/stretching techniques such as used by microtype.
Micro-typography is the art of enhancing the appearance and readability
of a document while exhibiting a minimum degree of visual obtrusion. It is
concerned with what happens between or at the margins of characters, words
or lines. Whereas the macro-typographical aspects of a document (i.e., its
layout) are clearly visible even to the untrained eye, micro-typographical
refinements should ideally not even be recognisable.
However, there don't seem to be any major changes to the package since 2021. (Which is not surprising, since it was first released in 2004.)
Interestingly, Guthenberg used very similar techniques with varying the shapes of letters so that the "blackness" of the page could be more uniform.