Toured in November, 2007. The Pantheon was one of my favorite buildings in Rome because of its age and history. The outside definitely shows its age but the inside does not. Just beautiful inside. The marble designs are wonderful and the colors are some of my favorites of blues and browns. Entry is free and photos are allowed. There are audio guides and I believe there are some guided tours (which I didn't have time for) so if you have the time and are a lover of history, that might also be worthwhile.
I'd also recommend going mid morning to early afternoon. There is not much lighting so if you go late in the afternoon or prior to sunset it will be very dark inside and you won't be able to see as well and will effect your photos. It's not that large inside. I think I had built it up in my mind to be much bigger given its place in history. Easy walking distance from other sites.
The Square outside has plenty of spots to sit and rest your feet while enjoying a coffee or gelato. A little pricey but you can't beat the view of the Pantheon.








