We went into the cathedral first which was free and had amazing art in it . The queue looked long but it only took about 10 minutes.
After the cathedral we decided to go up the dome. The queue was about 30 minutes and it was about €6 each (apart from children who were free). There are 463 spiral steps to the top, it is very tight and to top it off there is only one way up and one way down. By the time you reach the top you are almost on your knees getting onto the roof. I would definitely not advise anyone afraid of tight spaces or height to go up this dome, although the views from the top were magnificent!

Obviously, if you visit Florence you'll see the Duomo.
If possible, go inside. Entrance is free. You need sensible cloth like in any italian church.
Unfortunately, tourists can only wander on the back half of the cathedral, without chairs; and it's unpractical for looking at some paintings.
If you can, you might go up the Campanile (bell tower) - 413 steps and 6€ for an amazing view on the baptistry, the place and the cathedral.
You might too go up the Duomo - 460 steps and 6€ for an amazing view on inside paintings, the whole city, and how the duomo is built. There were more people over there, so sometimes you have to wait for several minutes and you could look around. Entrance is outside on the left of the church.

My wife and I made the climb to the top of the Duomo(which is even more work than we expected) but as others have said, it was really worth it. The views were so wonderful that we stayed up there far longer then I ever expected. I took picture after picture and now that I have all of them on my computer at home I wonder why I didn't take even more. I don't know when that has happened before. The day was bright and sunny but the sky was also full of puffy, litttle, white clouds being blown about. A sea of red, tile roof tops went out in all directions below us and it looked like we could see for many miles.

There are a number of things in Italy that may be worth missing but this wasn't one of them. We found this experience very special and if your up to making the climb and the weather is good, I think you will too.

Yes, it is a steep climb-but more than worth the effort. Try to climb the bell tower, too. Breathtaking views! And the frescoes are also worth looking at. Loved Florence-and all of Italy!