Pros
The atmosphere is very laid back, that's for sure. You don't have a manager breathing down your neck, babysitting you. Compensation is pretty good for full-time employees, especially the health care. If you're going to work at the Mayo Clinic in an IT position, don't expect to be too stressed out about work, or staying long hours as long as you're competent enough to get it done on time.
Cons
First of all, getting into Mayo Clinic for a full time position is extremely difficult if you don't have the right connections and lots of patience. The recruitment process there is horrible, as they may put you on hold indefinitely without giving you a word of what's going on. And, in order to get in, you must put on a fake show in such a way as to conform to their standards of personality and behavior. In my experience, not much work was ever really done there, but it was mostly meetings and other things that got in the way of real work.