We chose the 13.8 mile South Ridge II route. It was a glorious day without any weather, and the climb itself was rather uneventful except for a few efforts at route finding. If you're in a passenger car, parking before the 4x4 road starts, after you start walking on the 4x4 road you will soon come to a fork in the road. Stay on the 4x4 road to the right. Do not take the one to the left. This split is before the split that Roach references at 11,500 feet. Also, the Colorado Trail Initiative was camping nearby and they have done some excellent work on their trail that runs through parts of this route. The Colorado Trail is marked well with large cairns, but it took us a few wrong turns to realize that the large cairns are not for San Luis Peak until you get to the 12,620 foot saddle between San Luis and Point 13,155 feet. This is where you turn north to go to the summit, and there are small cairns along the way. Also, don't get fooled by thinking that Oregon Mountain to the east is San Luis.
Very nice climb. It took about four hours up and three hours down. Two others on top also chose the Willow Creek route, and one on top came from Stewart Creek. This is a log on top of the peak, but no sign-in at the start.