In the moment, I-Day is everything, because, at the end of the first day, it's all you know. It's startling, it's a shock, it's madness, it's emotional, it's mind-numbing . . . all at once. However, by the end of the first week, by the end of the summer, by the end of plebe year, by graduation . . . and the rest of your life, it really is nothing more than a blip or a blur. My memories of the day are more from the pictures my parents took than actually remembering the day itself.
IMHO, it's harder on parents. For them, I-Day symbolizes the beginning of true independence/emancipation of their child. It's all done in a single day -- hugging them at Alumni Hall as a civilian and, then, 8-10 hours later, seeing them as a member of the military. And knowing they won't see their kid for another ~45 days. And that their child is now truly an adult. It's a lot to process.
For both, it's kind of like jumping into cold water. Immediate shock and then gradual acceptance. It's just the memories are more the shock for the parents and the gradual acceptance for the mid.