[yaantc]

There a built-in similar function, `fill-paragraph`, tied to M-q.

I use it routinely and it's very nice. It has some language smart, and for example understand code comments, and can format comments nicely. It also has smart to recognize line header, and reformat while keeping those headers, with possibly a different first line. So if you introduce item (unnumbered list) with a "-" for example, you can either align the second line to the "-" or to the start of the text after, your choice. `fill-paragraph` will recognize this and stick to your choice when reformatting the paragraph.

I can't compare with `par`, which I've never used. But I'm a very happy user of Emacs M-q.