I spent a couple of weeks mooching around Corsica this September, mostly in the north, using the train, buses and hiking to get around.
I spent a couple of days in the the Cap Corse(the peninsular sticking up in the north) where I saw quite a few cycle tourers doing the coastal road. There's some nice remote sandy beaches up there but the coastal path around the tip can be closed if the winds are strong (as it was when I was there).
Calvi is more Riviera in style, beautiful sandy beaches around a large bay with mountains in the background but busy, touristy and more expensive. I liked Porto on the west coast - there's a great mountain/coastal road to get there from Calvi and Porto is in a beautiful setting with the Scandola headland and the Capu d'Orto mountain behind it. There were a few german roadies in the campsite there as you can head around the Calanche coastal road, down to isolated pebbly beaches, or inland into the hills.
Corte is the main mountain town in the centre of the island and has a great feel to the place - lots of locals hanging out in the cafes - and is handy for places like the Restonica Valley. I hired a hybrid bike there one day and rode around some of the mountain villages to the east - roads were empty. If I was going back with a road bike I'd like to do the D84 road from Evisa over the Col de Vergio (there's a tiny ski station at the top with a hotel and campsite) and down the other side - took it on a minibus and the views were great - a proper windy road with wild pigs round every corner!
I saw at least one mtb company in Corte advertising off-road guiding around Evisa so that area could be worth checking out.
I didn't get any further south so can't comment on that part of the island. Oh and in the middle of summer it's going to be hot, hot, hot!!
Some photos here:
Corsica 045 Spelunca Ota by , on Flickr