This is a feature that I'm quite sorely missing. I'd love to see it get merged soon!

I'll see if I can cleanup this PR in January. I've been quite busy lately.

Not the same, but related: using J to join lines in comments leaves the stray comment markers in the line. So if I have:

// foo
// bar

and I hit J in normal mode I get:

// foo// bar

Interestingly, :reflow handles both cases correctly.

A note regarding the implementation: vim also carries over the preceding whitespace of the current comment line.

/* Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do 
 * eiusmod tempor incididunt ut labore et dolore magna aliqua.
 *
 *       push   %rax
 *       jmpq   *%rax▎
 *       ▎           ^ 
 */      ^       if you press enter here
       cursor
     lands here 

This is very useful when writing comments with varying levels of indentation.

I'm also very excited about this feature! Just to get an update, if I understand correctly #8664 should do the trick (at least for line comments) but it's waiting on #4718 to get merged is that right?

Yep that's correct, we want to get in #4718 first and then look at #8664