In reply to AJM:
I've not climbed there in May but I live half way between Grenoble and Ceüse.
Early May can be glorious, with all the mountains coming back to life. But it can also be brutal, with snow making an appearance or lots of rain. So whatever you choose, I would be prepared to change your plans at short notice.
It should be easy though, there is a train that runs from Grenoble to Veynes-Dévoluy (great, dramatic mountain train journey) every couple of hours and from there you can hitch/taxi to Ceüse. Unless of course you hire a car, then its about 2 hours drive.