In ancient days, "circus" was the term used to describe the great stadium areas where the general public gathered to watch chariot races, athletic events, and other popular extravaganzas. When considering the ancient significance with the modern-day term, I'm not so sure that St. Peter's Square and Basilica in The Vatican might not also qualify as a modern day circus...without the three rings!
I'm not Catholic so I wasn't sure what to expect - religious or tourist experience? Aside from this being home of the Pope and a place seen often in various outlets of the media, this was simply one more stop on the jam-packed list of attractions I felt compelled to see while in Rome.
After poking around St. Peter's square which was heavily corridored off with only lanes for passage, I tried entering the Basilica...only to be sent back out and to a lower area right of the entrance. Everyone must check backpacks, purses, bags, etc. at no cost before entering so you might want to consider wearing something with extra pockets for camera, film, etc. they allow you to carry in. I also can't remember, but I think this same policy was also in affect at the Vatican Museum.
The Basilica could definitely win the title for "Church of all Churches" except it didn't have that feel once inside. The atmosphere was almost as chaotic as found around Piazza Venezia with the hordes of people milling about; even their quietest whispers and shufflings echoing within the cavernous interior. This may all be sounding disrespectful to Catholicism, but I found the indoor "circus atmosphere" even more disrespectful; especially with smaller mass services being conducted in various chapel settings.
I came here after touring the Vatican Museum so my mind was still blurred from "art overload" not to mention my lack of knowledge of Catholocism. But for those who do, there's a gazillion things to be seen. Frescoes inside the smaller cupolas and the great dome are definitely worth looking up for, and other works of Michaelangelo, Bernini, and noted artists are scattered about...though darkness makes flashless photography all but impossible.
There was quite the line waiting to descend into the grottoe-burial areas, but don't let it discourage you. Once below, the areas open up allowing you to leisurely or quickly pass the countless crypts, tombs, prayer altars and such including where St. Peter, himself is presumed to have been buried.
I can't imagine having came to Rome without touring The Vatican, but I didn't expect to leave with a "Been there - Done that" type of feeling either.