Anyone going to Rome--if you are there on a Friday, definitely do the Vatican and museums for the night hours. You can get reserved spots through an online reservation.
We first entered the basilica around 5:30 with only about a 10 minute line at security. The church was not too crowded, but there was a 5:00 mass so the apse was not accessible. Alternative--go to the 5:00 mass.
We had 8:30 reservation for open guided tour. We walked straight to the front of the line and they let us in at 7:45. Most people were gathering with groups but with Rick Steves audio guide downloaded, we cruised right past them. Even so, no lines anywhere. It was like a dream. Sistine Chapel was probably the most crowded but we all were able to get a seat within the 40ish minutes we were there.
Finally, the dusk light coming in through the galleries was beautiful.
I can't recommend it enough!