I tried having a good mix of letter writers. I did have a TA write one, but she had known me very well for the last 2 years and could talk about me in more specific terms. I also had full professors write letters for me, I'm sure they didn't have the level of familiarity with me that the TA did, but at least the medical schools had a letter from someone who has been successful in the field and seen many students come through his classes to say how I performed, especially in comparison to others he had seen. So this was a strategy that worked for me, not to say it's the best way to do it, but I found it really hard to get letters only from professors and tried to make it work.