I had a really good experience at Stanford. I did 3 weeks of peds anesthesiology, one week of which was inpatient peds pain. It was somewhat loosely structured in that I got paired up with one of 4 or 5 residents each day, but I sought out enough consistency with a couple faculty members that I felt comfortable asking for a letter at the end. Dr. Pearl also makes a point of meeting with each visiting student to discuss their application and future in the field. This was great for me because I came from a medical school with a really poorly regarded program, so this was really my only mentorship for the application process. The hours are what you'd expect at a top program. I arrived around 6 and left anywhere from 4-8pm. The residents were generally smart and excited about having students around. A couple made the point that they particularly liked visiting students because stanford students were a little spoiled and didn't like working hard.
Not sure which program you're coming from, but my advice is definitely to do an away rotation somewhere. It'll show a lot of initiative, interest in the field, that you're tough enough to pick up and go somewhere, and it's an opportunity to get a letter.
