We organised our trek very last minute and of the various tour operators we contacted, Inca Trail Reservations were by far the most helpful and responsive. Their online booking system was extremely easy and clear and the personal online space to which they uploaded our various tickets was really helpful.
As for the trek itself, our guide, Carlos, was excellent - very friendly, fun and clearly very knowledgeable. The sleeping tent were spacious and sleeping bags (which we hired from them) were super warm. Being a vegetarian, I was nervous about the quality of food I was going to get. But I needn't have worried; the vegetarian food was plentiful and, most importantly, delicious, with 3 full meals (plus a tea time snack, usually popcorn plus something sweet) every day.
The scenery is spectacular, particularly Day 3 and on Day 4 when you get to the Sun Gate and see Machu Picchu for the first time (you won't get this sense of awe if you just do a day trip from Aguas Calientes). However, be prepared for a tough time on Day 2; the trek up to Dead Woman's Pass is long and very arduous but the guides are very patient and encouraging.
Overall, I would highly recommend the 4d / 3n trek to Machu Picchu (Days 3 and 4 totally make up for the pain you suffer on Day 2) and would definitely recommend booking your trek with Inca Trail Reservations.