We have been three times in Corsica, visiting the center, North, and South of the island.
The first time, the island was experiencing one of the hottest months of June for a long time. Some streams were already dry and the temperature along the coast was nearly unbearable.
Fortunately, Corsica is a "mountain in the sea" with summits above 2000m (6000 feet) and we spent the first week of our stay exploring the remote villages and valleys of the center. This encouraged us to visit it earlier in 2003 and 2010. Late Spring is the best period to visit the island as it is full of flowers.
We are still planning to return.