Susan C.
Yelp
Since this is basically the number one tourist destination in Milan, there's not a lot that hasn't already been said. Certainly worth checking out for historical, architectural, cultural, religious reasons or pure curiosity!
Tickets: there are about half a dozen options for tickets on their website duomomilano.it. There are other 3rd party resellers but why pay an upcharge when you can go directly to the source? Some options include Duomo (and duomo museum) only, rooftop only, Duomo and rooftop with either walk up or elevator, Duomo and Ambrosiana (another museum/library with some artwork by Italian and Dutch masters, and St. Sepolcro crypt). They don't give many descriptions of what's what, so do a little google research to determine what works best for you. All are timed entries though if you come within the 15 minutes give or take that seems to be fine. Audioguide and walking tours are separate.
Entry to the Duomo lines up to the right hand side of the building (facing it from the main piazza), if you didn't buy tickets in advance the ticket window is next to the entry line too. Def recommend getting there early if possible, or later toward evening to avoid the most crowded times, though the lines move fairly steadily.
If you go up to the roof, the roof entry is different--around the left side of the cathedral. The stairs--in the form of a spiral stone staircase inside a stone stairwell--are fine if you're in moderate physical condition, though elevator is faster. For stairs it took about 10 minutes to climb to the top at a steady pace with a couple short pauses. The views are beautiful even with the ongoing renovation, you can see for miles around on a clear day. Note that the access to the rooftop is directional, meaning once you walk past the initial section of spires/flying buttresses and ascend to the roof proper, then you can't go backwards. Same once you descend from the roof. Understandable since the stairways up and down are narrow so they need to keep the flow of traffic moving.
Inside the cathedral I was surprised to see they let tourists freely walking around the outer perimeter of the central pew area during Sunday Mass. I appreciated that they let visitors sit thru some of the Mass since it's not something I've done before in Italy. The main seating is cordoned off, but visitors can walk around and to the back of the cathedral for pictures, etc.
The entry to the archaeological area is along the wall between the front entry and exit doorways. There is a sign and attendant checking tickets. The open area under the church is small but cool to see the remains of the refectory that used to stand in this location.
The duomo museum has a separate entrance line and ticket office and is located in the Palazzo Reale which is next to the main duomo building. Ambrosiano and crypts are separately located a walkable distance 7-8 min away from the duomo. If you get the total package you can easily spend between 2.5-6+ hours between the different buildings and museums.