Been over 4 weeks since my second, all day long interview and either a) the company hasn't made a decision, or b) they did and the recruiting agency is just moving on. This person really wasn't "my recruiter", but more of a headhunting service that this particular company utilizes all the time. I'm trying not to bug the recruiting agent, as I did follow up with her about a week after the interview, but closure would be nice if that's the case. Either that, or the company is still interviewing and I'm simply "still in consideration". But after this long, I'm not sure how good those chances would actually be at this point.
An initial "Thank you for the opportunity to interview" is necessary to show basic thoughtfulness and appreciation for the interviewer's time and energy. Beyond that, and in a brief, pithy format (no longer than approximately three to five paragraphs), you'll want to address any overarching points that were made in the interview. For example, if a specific question or discussion point stumped you or left you feeling a little less than articulate, then use the follow-up letter to smooth it over.
Show, through word clarity, that you are qualified by pointing to a story, result, or skill set that will help overcome the objection you may have left as residue at the close of the interview. As such, consider the follow-up letter an opportunity not only for professional courtesy, but also for clean-up, if you will, as well as to reinforce your value for the specific role.