Pros
I had an amazing time at O.F.A. people from all walks of life, around the world put their lives on hold to volunteer, intern (and some were paid FT employees). Also, of great value, the cross-training turned out to be invaluable (for me personally) because it aided me in changing my career focus. Once I am re-established I would consider volunteering on one of their future projects.
Cons
No real downsides -- just wish I had been able to hop on board two years earlier!