Pros
An advanced degree in academia from Harvard is without question prestigious, and the resources to focus on just about anything are available. Even if you have a poor time here, that little bump on the resume might open a few doors. Your peers are uniformly intelligent and interesting.
Cons
If you are truly interested in mentorship from a professor, this is not the best place to be. Professor's attitudes range from the arrogant to quite friendly, but almost all of them will not have the time to truly mentor you (with some exceptions). If my entire entering class had attended a lower level institution, I imagine more of us would be going into academia after our degrees. Finally, those smart peers can be a spectacularly unsupportive environment if you're susceptible to that kind of thing. Again, it's not malice, but everyone feels a little insecure about their standing, and that can undermine a lot of interactions.