I have to declare that I have completed the MBTI accreditation and do selectively use the instrument in support of some of my consultancy work. I find it quite useful in drawing out some of the differences that people display particularly around communication and mostly with regard to workplace harmony. (Mind you, I make no commercial gains conducting them as I my work is internal).
I take on board the issues raised in Skepdic but I also feel some sympathy for the readers. That being said I might take the liberty of offering my own $0.02.
The use of the instrument as an absolute or final say in something as important as whether or not someone gets a job is quite abhorrent. I use the dichotomies that the instrument draws out merely as a means to draw out the normal variations in individual preferences and then as a platform to discuss group preferences.
The main criticism that I personally have heard about MBTI is that people use the instrument as an excuse for the worst examples of behaviour attributed to the Type. Unfortunately, this has been a behaviour that I have witnessed myself. That being said, a well administered MBTI session (i.e. not one online, which lacks the various stages required to be anywhere near useful) should show the participant the ways to realise their strengths and manage their weaknesses, no matter how they are derived.
And how they are derived are hardly that concerning when someone identifies as an Extravert they might be more comfortable in large noisy groups and their weakness might be that sometimes they exclude those who identify as an Introvert... sounds fairly uncontroversial to me