We get some people who will return to their asked question to post "this is what I ended up doing". Some update their original question, some post it as an answer.
When I see it as an update to the original question, I've always posted a comment thanking them for the update, then asking them to make an answer out of it, since "this is what I did" seems like an answer to "how do I do this".
This question is a prime example of the genre. However, as of this moment, this is the very next question in the queue, and our esteemed moderator Niall C.♦ has, in just the last few hours, deleted the self-answer by the OP and made it an update to the original question. Granted, in this case, the OP did ask an additional question in his answer (which should, of course, be a new question).
It, obviously, seems to me that the "this is what I did" update is actually answering the question and should stand alone as an answer. However, that's been countermanded by a mod who, I presume, knows the SE and community rules better than I.
My question is which is the preferred or official method of dealing with a "completion" update from an OP?
I don't think we'd want to discourage people providing updates, as they do provide closure and it does help others to know what someone else actually did with this information.