I've said a similar thing in my other reviews of the area. If this cathedral were anywhere else, by itself, it wouldn't be as impressive. However, with the backdrop including the leaning tower and baptistry, it makes quite the trio.
I was impressed with the inside of the church, especially the carved pulpit on the left side of the sanctuary.
Entrance to the cathedral is free, but you do need to go to the ticket office and get a ticket to get in. (That way they can track how many people are visiting.) With the rest of the plaza, it's worth a look inside.