With little NE wind you can have soaring on the summit, pretty.
Beter season: automn (september to november), before snow.
You can have good XC from this site, but it's not the spirit if you walk 3h ;-)
Access
From Grenoble go to 'Le Sappey' village and cross it. Follow the D512 road to 'Saint Pierre de Chartreuse' but turn to the right before it, 'Saint Hugues' direction.