In reply to birdman:
If you are doing the trail with a guided company check them out before hand. There are many not so ethical companies that will charge you what you would consider a fair price, but who will not be paying a fair price to the locals who will be carrying the equipment.
We went in January, which is the middle of the Peruvian summer, and at 4000m it was pretty cold, and drizzling quite a lot, so some form of waterproofs definitely recommended. I'm not sure what it would be like in May, though being their Winter would imagine that it would be even colder.
The tourist train from Cuzco, is one of those great railway journeys of the world, going through some amazing scenery, and for that reason I personally wouldn't bother with the Inca trail, and use the 3 days you save to explore other parts of Peru, like the Cordillera Huayhuash or Blanca.
I agree with Alex above, that Machu Picchu is one of those awe inspiring places, that everyone should try and visit at least once. It is truly amazing.
One other thing, when we went you can't pay by credit card to get in. We made the assumption that you could, and finding out you couldn't scrimped our money together to find we were about 10 dollars short, and ended up having to borrow some money from a local to get in!