Letters from program directors are a nice little ego-boost, and probably do give you an idea (for the most part) if a program is interested in you. I suppose it's possible that a program could send letters of you're-so-awesome to everyone they interview, but I haven't heard of that actually happening anywhere. Just rumors. So assume the letters are a good sign.
What I have heard of (and, heh...been the recipient of) is people failing to match to programs that sent them a "we like you" letter. So don't think that just because you got a letter, you'll for sure match to the program. Remember, a program that matches 5 applicants might send out letters to their favorite 10 (or however many - I don't know how many is typical per program; some don't send any). It's the same with applicants...plenty of applicants write letters to their favorite three programs, but will only match to one.
So even if you get glowing letters from your top three picks, RANK EVERYWHERE YOU WOULD BE WILLING TO GO.
*ahem*
(Sorry about the all-caps. But every year, applicants are told the same thing, and still some people don't listen, botch their rank list, and have to scramble. Yes, it could be you. No joke.)