I decided to hike the Inca Trail because I had friends who had done it and raved about it. I hiked Kilimanjaro a few years ago and it was a top notch experience. This experience was fabulous but it didn't quite meet Kili standards.
The food was magnificent. The porters worked extremely hard and our guide was impressively knowledgeable. The views were breathtaking despite being socked in by clouds on the third day. It was a considerable amount of hiking (especially hills!) so if that is what you like to do, then this is the trip for you.
After hiking to Machu Picchu, it's still worth continuing to go on up Huaynapicchu as the views are awesome and the hike up is not too scary (and that is coming from someone who is afraid of heights!)