I got kind of screwed by my advisor who was writing the letter from my school's pre-med commitee. He apparently put like zero effort into it even though he knew me. I never saw it of course but I had schools tell me that given my scores I should have had more interviews and earlier interviews so I know that there was something wrong with that part of my application. Anyway long story short I got waitlisted by Emory and Washington U post interview. I think I would have had a decent shot at Emory b/c they requested 2 additional LORs, it was just too late b/c my advisor had gotten around to sending my LOR 2 months after I'd asked for it so my interview was really late. I didn't end up pursuing either Emory or Washington because I decided to go to my state school and pay 1/3 of the tuition. Its a good school too.
So short answer, yes, you can get in to a good school with a mediocre or in my case $hitty LOR, it just might take a little more effort to make up for it. Also if you do get waitlisted at a school you really want to go to you can send letters of intent along with more LOR's. That seems to make a big difference at a lot of schools. Anyway lots of luck. Hope your process goes better than mine did.