We live in Northern California, so I can't speak to how the FlexPass compares against the SoCal-specific passes, but I was thrilled to see this pass introduced and we are planning to become passholders for the first time this November by buying the FlexPass. To me it is a no-brainer compared to the Deluxe pass. My kids are school age, so pretty much the only times we can go are times when the Deluxe Pass is blocked out. The Flex pass at least offers the possibility of being able to go during weekends and summer and from monitoring the big FlexPass thread, so far there haven't been any complete unavailability days, though obviously that could change as more people get the pass. Compared to the cost of parkhoppers, the FlexPass pays for itself if we make two three-day visits in a year. In our case, we are buying a three-day parkhopper to upgrade to the pass, so we still get the Magic Morning for the first visit. I can't imagine the availability will be so low that we won't be able to get reservations for our second visit (which will be in February, when my kids have February 10th off school, which is a good to go day, so we would be guaranteed three days if we can get reservations for the 8th/9th). Anything beyond those two visits I'll consider a bonus.
If you live close enough to visit on the fly, I think this pass would be an even better option.