We visited the Vatican on the last Sunday of the month when the entrance is free. If it is a big family group then it is worth it but it is obviously very very busy and the queues extend right round the Vatican. It took us over an hour before we could get in but it was worth the wait and it is the best thing in Rome. Normal days when they charge entrance it is a short 20 minutes queue.
It is, however, open for half a day on the fourth Sunday of the month. The last entrance is at 12:30 (on the day we were there) and the museum closes an hour after that.

Beware of people offering you guided tours and offering to take you in straightaway without having to wait in the queue. They have people standing and waiting in queues and then get you to join them, essentially queue jumping. They charge you £25 euros per person for helping you jump the queue. It is distasteful and if the vatican staff see you they would tell you to go to the back of the queue and you would have lost your money too.
Inside it is all beautiful with the painting on the walls and the ceilings. You can spend as much or as little time as you want walking through the various galleries.
St. Peter's cathedral is well worth a visit too. My children went up the cupola and said it was amazing and worth the climb of over 500 steps, part of it just holding on to ropes

Vatican City is immense. I had no idea it had so much to offer. It was an expensive entrance fee but so worth it. Bring comfortable shoes and a camera, but watch out- picture taking is not allowed in many areas and you'll get kicked out if you persist. I loved Michelangelo's Sistine Chapel. My husband loved the many galleries and domes covered with amazing paintings. Our daughter thought the mummies in the Egyptian Museum were fascinating. Allow many hours and spend the money on the audio tour. The statue of Mary holding Jesus- the Piete'- also by Michelangelo, in St. Peter's Basilica is breathtaking no matter what your faith! This is also where you can attend a public audience with the Pope...

Had a great tour of the Vatican with Angel Tours Jimmy from Ireland. Very informative and we saw optical illusions in the tapestry room that the other tours just marched right on by. When our tour was over we contiued on to climb to the top of the dome. Our son told me there was an elevator. He didn't mention it only went half/way and I am afraid of heights. What a view. Only good for the fit, the climb was awsome and narrow, bending around the curvature of the dome in a very narrow space to end at the very top for an incredible view of all of Rome. Pat

We stood in a long line for a long long time. i don't know if it was the waiting or the reputation of the sight, but I almost was disappointed. Too many people, difficult to see things. Maybe it was just too magnificent for me!