Any kind of personal story is good. Also anything that makes it less boring. What you want to avoid is something that is restating information that is elsewhere on your application (ie. student is in top quartile of his class, scored x on boards, etc).
Opening paragraph -- Brief explanation of who letter writer is and relationship to you. "I write to enthusiastically recommend xxx for urology residency. I am the director of urology at yyy facility. I worked extensively with xxx during his rotations on urology as well as on a research project."
Next 2 paragraphs --
Explanation of why you were great. "While on service xxx exhibited compassion and great work ethic. He was a great team member" Ideal place for some kind of anecdote of a time when you went above and beyond on service
"XXX's role during the research project was to..." Ideal place for something about your research skill and/or writing abilities.
Final paragraph --- Conclusion about strongly recommending you. "I recommend xxx without reservation, and we are actively recruiting him to stay at our program."
Anyway, hope that helps.
