The Sierra Century is my favorite ride. I've done the 100k or my own 80 mile route many times, and the 100 mile route the last two years. Unfortunately, I've bailed on Slug Gulch both times. Everyone tells me Slug is doable, but at about mile 80 on an already hard ride it just seems like too much. Maybe this year... Yellow has done Slug Gulch so may be able to help you there.
Back to the rest of the ride. Gorgeous scenery, great food, the weather always seems to be good. My rating: 5 chain rings!