we had a lovely time, I am not convinced that we needed to book ahead as it wasn't very busy on our day but I can see it can become busy and it may well be neccessary to prebook and skip que. There is certainly lots to see and take in be prepared to walk with a lot of people in some very tight areas. Lovely statues, and many many painted ceilings and walls. A must see for anyone who is into arts.
Make sure you visit the Basilica in St. Peter's square as this is also a fantastic site. as well as the square itself.