Definitely a once in a lifetime adventure. It's amazing walking in the footsteps of the Inca's who used the trail for religious pilgrimages.
The hike is not for the faint of heart. I'm in fairly good shape, but the trail destroyed my knees, by day 4 I could barely take the steps. Don't let them fool you, day 3 is worse than day 2.
So prepare yourself mentally. The guide was amazing. He was very knowledgable and constantly stopped to tell us the story of the Inca's and even teach us about the plants and their ancient and current uses. The food was also amazing, I couldn't believe the variety they offered while on a 4 day hike.
I definitely recommend Inca Trail Reservations .Carefully read their page and make sure you pay attention to the packing list.