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).
In my experience, thank you notes generally will not be the thing that leads a firm to give a candidate an offer, but they can result in no offer or a pulled offer - for example, when candidates inadvertently include typos or misinformation in their thank you notes or call a firm or even an interviewee by the wrong name.
My advice would be to limit thank you notes to in-person interviews (not phone or video screens) and to only send them (1) if doing so is a natural inclination on your part and/or (2) an interviewer or recruiting coordinator has gone above and beyond to meet with you, rearranged their schedule to accommodate you or has provided you with details and information that you find to be exceptionally invaluable and informative.
Keep in mind that your recruiter can and should de-brief with you following an interview and can thereafter convey your appreciation of and continued interest in a firm on your behalf, thereby relieving you of the need to directly convey this information to your interviewers. If you are not working with a recruiter and feel compelled to let a firm know of your continued interest, or have additional details to convey about your experience and candidacy, you can always send an email to the person who organized your interviews for you (which is typically the firm's recruiting coordinator).
If you choose to send thank you notes following an interview, the following tips should help you to avoid potential pitfalls and post thank-you note fallout: (1) limit thank you notes to in-person interviews; (2) do not be formulaic, generic or insincere in tone but instead reference specific discussion points and aspects of the firm/practice group that appeal to you; (3) make sure your thank you notes contain no spelling or grammatical errors; (4) keep the note short and sweet; (5) if you are drafting thank you notes to multiple interviewers at the same firm, be sure to tailor each note to the recipient and base it on your specific conversation with that individual; (6) be genuine and let the recipient know what you got out of the meeting and that you appreciate them taking the time to meet with you; (7) conclude your note by indicating that you are happy to meet with additional attorneys and/or answer any additional questions the firm might have; (8) be professional but be yourself; and, lastly, (9) be accurate in the details you articulate about the firm and the content of your meeting.
Finally, I would recommend that thank you notes only be sent if you are indeed interested in the position you interviewed for. If, following an interview, you conclude that you are no longer interested in the position, then do not send a thank you note as this will send a mixed message to the firm.