Even after visiting Machu Picchu, I still don't know what it actually was. I heard about 10 different theories from very knowledgeable guides (and here's a great National Geographic rundown). No one really knows but it didn't take away from the sheer awe that hit me on my first glimpse of the panorama. What I didn't get from my sixth grade textbooks and photos is how high up it is and how stunningly gorgeous the surrounding mountains are - and how you feel like you can almost touch the sky. What I recommend is hiring a great guide (e-mail me for my favorite, one of the best guides I've ever had!) but also leaving at least an hour or two for alone time - to just sit, stare, and soak in that you are actually there. After long plane rides, connections, a train ride, and a bumpy bus ride, you are THERE. And it's better than expected.
If I go again, and I hope to with my future family, I will still take the Hiram Bingham train, but I will also spend one night at the Machu Picchu Sanctuary Lodge, the only hotel on site and owned by Orient-Express. I had tea there, and took a look around, but you have to book many months in advance. There are only 31 rooms, and because of where it is and protection rules, it will never expand. I also want to get some hiking in, though it's not for me to do the four-day Inca trail hike to Machu Picchu.