I have emailed a few, addressed to both PC and PD. One PC replied back saying thanks and that they are still considering. Three PD's have sent what look like personal replies stating that interviews are full, but that they'd get back to me.
My suggestion on content is to keep it short and simple. Keep in mind that EM docs, in general, appreciate getting to the point in as few words as necessary. In my letters, I pointed out a particular reason I liked their program, but only spent about 1 sentence doing so. I was also straight forward, stating that I hoped they'd consider me for an interview.
This is just my logic paired with advice from a mentor. Keep in mind that I am a fellow applicant with no real basis to give advice.