Most people agree that, in today's modern, technologically driven society, thank you emails are preferable to handwritten notes. But where the controversy stems from is whether or not candidates should send thank you notes at all following interviews.
There are many competing views on this subject, even among different BCG recruiters and, indeed, there is no right or wrong answer to this question.
I generally advise my candidates to do what feels natural and right to them. If you are the type of person who would typically send a thank you note to someone for taking the time to meet with you, then there is no reason to forsake your usual instincts in the interview context. If, on the other hand, sending a thank you note is not a natural inclination for you but, rather, something you feel compelled to do for the sake of protocol, then I would recommend thinking twice before sending a note – as you may do yourself more harm than good (for example, by sending a note that sounds insincere or generic or formulaic).