Here's the thing - you need timed entry tickets now and everyone books the middle of the day like an idiot. Book 9:30am or 6pm slots. Early morning you get decent photos without crowds, evening you get sunset light on the mosaics.

If you're planning to visit anyway, the paid monumental zone has the famous mosaic bench and salamander, but honestly the free areas have better panoramas for photos. And for the love of god don't try to drive there - parking is a nightmare and the bus drops you right at the entrance.

Most importantly: it's not worth more than 90 minutes max whether you pay or not. I don't care what your guidebook says.