Hands down the most beautiful artwork I have witnessed outside a textbook. Additionally, this is the most stunning religious building I've ever visited (and I've visited the Hagia Sophia and other beautiful mosques of Istanbul).
The outside of the cathedral is pretty, but nothing amazing. But the altar will simply take your breath away. I think anyone who appreciates beautiful, ornate detail will thoroughly enjoy this church regardless of his/her religious affiliation.
You can pay $5.00 to tour the church, or stay for a mass and check it out for free.