Law2Doc said:
A letter itself per se can help you but probably not get you past "below average" stats. Almost everyone applying has good to great LORs, or they wouldn't have asked the writers to write one. Using pull from someone with strong connections by means of a LOR might make a difference though.
For the most part, LORs are glowing, yet very trite and impersonal.
If you get a letter that says "this student is very hard working, he'll be a great doctor!" then they'll just look at it and it will effect you neutrally (this is only because most LORs say the same thing.)
Unfortunately, I think that since most LORs are glowing, a bad LOR can detrimentally affect you....fortunately, these LORs are rare, particularly if you choose your recommendors wisely.
Finally (which seems to be the situation in your case), if you can find a recommendor who genuinely knows you and writes a unique or distinguishable letter about you, then it might be enough to push you over the edge...but let me emphasize one thing: it will usually do only a little bit of good granted your recommendor isn't Hemingway.
Bottom Line: LORs kind of suck. For the most part, they have no effect since everyone has glowing LORs (yet they sure are a pain to get). Furthermore, a good LOR can only help you a nudge and a bad LOR could kill your chances.
-Dr. P.