I've ridden up and down the 15/Cajon several times. Coming down isn't much fun as you constantly have to bunny hop and dodge debris but it's certainly doable. Going up isn't so bad but you have to deal with the constant loud noise and smell from the freeway as you suffer up the hill. I prefer Summit Valley and 138 to the Cajon.
Do as LCI_Brian wrote, "...road on the east side of the railroad tracks from Hesperia, which becomes Summit Valley, then right on 138, then 15 to Cleghorn. From Cleghorn, you can take the old Route 66, then it's side streets through Devore, then past the Glen Helen Pavilion. But then the road is washed out from Glen Helen to Summit, so I've taken the 15 instead without any problems."