As the previous poster already mentioned, GR 20 is known as Europs toughest long distance trail. And it is tough! Most days will have you climb up at least 600 meters, but often far more then that, and then descend at least 600 meters again. It will give you blisters, your knees and ankles will become minced meat, and the sun and heat will nearly kill you. You'll be constantly hungry, tired and dirty. But it will also give give you pride and satisfaction! You will see extreme beauty and get to know yourself and you partned a lot better!
One word of caution - it is not for people without previous mountain experience! There are a lot of scrambling sections, the most notorius being Cirque de la Solitude, where a fall from the wall might in worst case kill you. You have to be used to walking long distances with a heavy rugsack and know how to decend on scree without faling or twisting your ankle. And you have to love mountains to be able to endure all this!
The refuges on the trail offer food, so you don't have to carry anything else then water. But breakfasts offered there are just a cup of coffe, tea or coccoa and bread with jam, so if you need bigger breakfast, take it with you. Dinners on the other hand are sufficient, and you can even have wine (well, not sure if it deserves this name, but it is red, with acohol ad made of grapes, haha).
Since July 2009 you have to reserve both a place in the refuge and a place to pitch a tent through the national parc web site or in the office in Calenzana or Corte.
We started walking on June, 10th, so there were just enought people to be social but not so many as to be crowded, but later in the season the GR gets very crowded.