I truly believe that during your personal time you should display your absolute true colors, however extreme. Therefore, don’t get me wrong, you should fully explore your personal style and I am fully supportive of the arrival of the male bun!
That said, what you do, wear or portray in the office is an entirely different point. There are times when you are building your career were it is important know your audience and tame your appearance.
Corporate America is still a conservative boys club and you must choose your attire carefully, thus I don’t personally feel a bun will portray the correct image. If you are working in the music industry or some modern advertising agency you may get a pass but law firms, public accounting, hedge funds and banks will not see this in a positive light.
David Beckham fails to disappoint in any sense and is a true trend setter. If you are an international footballer (with the benefit of being tremendously handsome) then you can of course carry off such a look. In conclusion, sporting this looking in a casual setting is perfectly acceptable and David gives you the perfect example.
Moving to the more formal look then I think it should be avoided. During your career you need to be taken seriously and there are some that will look badly on such a sense of style.
Bottom line, choose your time to sport this look and be careful.