The splendor of totality
You cannot completely prepare yourself for the sight of a total solar eclipse. When totality arrives, you will experience primal emotions and wonderment at the unspeakable beauty of the corona and the panoply of colors and light as you've never seen before. You will involuntarily scream, gasp, or perhaps cry at this astounding vision.
You will be looking straight down a billiard shot of the Earth, Moon, and Sun lined up perfectly. By celestial coincidence, the Sun and Moon's apparent disks in the sky are nearly the same. This serendipitous fact allows the Sun to be completely covered during eclipse yet also allows us to see the Sun's atmosphere, its corona. The corona is hidden from us all our lives except for the very special moments during the total solar eclipse.
When you look at the eclipse, you will perceive the blackest black imaginable surrounded by the Sun's ever-changing atmosphere, the corona. The quality of light is stupendous with an amazing light show of iridescence, scintillation, and delicate colors.
Another realization that may hit you during totality is that you are watching the solar system in motion. In real-time, you can perceive the relative motions of the Moon around Earth and the Earth around the Sun.
Even for those who have seen it before, a total solar eclipse is an intensely emotional experience. You will feel ecstasy, wonder, and regret when it is over. You will immediately discuss plans to see the next one.
A piece of wisdom that all experienced eclipse observers tell those new to totality: do not attempt photography during the precious two minutes of the Sun in total eclipse. Instead, capture the moment with your senses and savor each second of the precious two minutes of totality. What you see will be seared in your mind's eye for the rest of your life. No camera can capture the full range of light, darkness, and colors of the corona and sky. Besides, you will be so mesmerized by the spectacle that you may not be able to operate your camera.
Experienced astrophotographers will make their photographs available on the web soon after the event. You will appreciate these photographs but with a knowing smile that these images, no matter how good, are a pale reflection of what you saw with your very own eyes.