I spent 4 weeks at Mayo this past summer. The residents were very nice and extremely helpful. The great majority of the faculty were open to questions and would teach.
I spent each week on a different service, each of which was manned by 2-3 attendings. Unfortunately, there was only one attending who allowed me to see patients on my own. The others had me follow around the residents, which was okay, but just watching them at times became boring. This also prevented me from spending enough time with any given attending to ask for a meaningful LOR. There was ample opportunity to observe surgery on each of the services.
The residents seemed very happy for the most part. I was told by one attending that doing a rotation there means a lot when it comes to the application process. The facilities seem to be top notch, although I was surprised to see they still use film for fundus photography.