I did one once under an unusual circumstance (paper passed peer review but... maybe it shouldn't have..., and rather than reject a paper that technically passed through a fair but bad draw of reviewers, the editor solicited some response papers). It was easy, and basically I just meated-up a normal review with citations. I don't think it's very common anymore; I cannot even recall them other than for special issues in which they are written by someone asked to summarize/integrate all the special issue papers. I think those get cited a respectable amount.
I did. It was fun. Split it with a couple Co authors and limited ourselves to one response (could have become an argument). I can't know for sure but I think it was good for visibility.
I've been invited to write a couple of commentaries. They were both the result of a peer review process, though one was about a specific study and the other was more topical. Mine appear in medical journals, where they seem to be relatively common. They have been cited once and twice, respectively. But they were pretty easy to write, and it's not every day that a journal editor offers you page space to opine with relatively few constraints.
I've never done one, but I look forward to the day i do. That's one of my academic dreams, to be able to share my thoughts, critiques and comments without having to provide data (citations, sure, that just makes sense).