In reply to Tony the Blade:
Yes, probably,as long as you think you will have time to go out and buy the 1:25k maps once you are there. I knew we didn't, so got the ones we needed before going. But then, we knew our route in advance and had a tight pre-booked schedule. We also had the rough Guide 1:135k map for the whole island to get an idea of the general geography.
For the GR20, we used the Paddy Dillon Cicerone guide, but the German and French guides, available in the local shops, have better maps of each stage.
I particularly liked the Pietro Piana refuge. We had to bail there because of bad weather (we had three teenagers with ius) , and walked down a very lovely valley to the railway at Tattone/Savvvagio, where there is a camp site that serves great food, past the fantastic Bergeries de Tolla - for cheese, bread, sausage and wine. We spent a night in Corte then took the bus up to Col Vergio and rejoined the GR20 there. I recommend a scramble up the Paglia Orba if you stay at Ciottulu di i Mori.
Calvi, at the northern tip, is rammed with cafes etc and a bit of a let down after the interior.