It is an interesting question, we all chase after the big monuments and then have to share them with thousands of others. The question that has always played on my mind has been, am I not better heading for the under-visited alternatives. For example, in New Zealand, Milford Sound is undeniably beautiful in that rugged fjor-land wilderness kind of way, but then loads of people head down there and you have to share the view. Last time I visited I tackled Gertrude's saddle, a stunning 4 hour trek that takes you to a stunning viewpoint over the fjord without any of the crowds. Perfect.
So back to Machu Picchu, is it worth it. Heck, people even head out to Peru solely for the purpose of a Machu Pichu holiday. Well there are alternatives. Choquequirao is known as a sister site, and whilst it isn't quite as dramatically situated it is very rarely visited the trek there is incredible and well having it all to yourself is a big deal after the crowds at Machu Picchu. But then, do you really want to go to Peru and not see Machu Picchu?