Correct use of this word normally indicates that the speaker/writer has attained enough education to be able to distinguish between the objective and subjective cases. Incorrect use brands the speaker/writer for all time as a pretentious wanker.
An example of the former would be: "To whom are you referring?" A classic example of the latter is: "Whom shall I say is calling?" (or is it? - this week's pedant's challenge)
Many argue that words like 'whom' which people find difficult to use correctly should be done away with, in the interests of simplicity. For these people I have two words: newspeak.