I've received 8.35" of rain in my back yard this year. Since I have a few years worth of data on rainfall in my back yard, and decades of data from the nearby Los Angeles area, I feel that I can draw a few conclusions.
1. Rain barrels still suck. I've said this before and here's a short synopsis of why with the caveats. This last winter we had 17 storms come through that dropped measurable precipitation. That's 17 opportunities to catch rain water. The typical home rain water harvesting setup involves 50 to 100 gallons of storage. So that's perhaps 1700 gallons of rain water you could have saved and used after the storm has passed. Go figure out how much that actually saved in money versus the cost of the installation. I've done that already for you, and it's not worth it. Wouldn't you like to use that water during the summer on your vegetables? Don't bother, you can't realistically save 50 to 100 gallons for that long.
Caveats: When storage is abundant (several times the amount of rain that your roof sheds in one storm) then it might make sense. There are under-house bladders and cisterns that seem to meet that criterion. When storm frequency is greater then it might make sense. When storm season is longer it might make sense. If you have occasional summer storms then it might make sense. The climatic conditions aren't likely to change enough in California even in the face of global climate change. Really, don't bother with a mere barrel or two.
2. Rain water retention still makes sense. When you are storing the rain in the ground (eliminating run off using swales, or rain gardens, or permeable hardscape, or however you do it) then it's easy to store large amounts and you don't have to fuss with infrastructure costs or maintenance other than your garden.
3. This winter was typical for Los Angeles. Everyone's crying drought, but the native plants living locally got a normal amount of rain. Do you think this is surprising? The most frequent amount of rainfall that the greater Los Angeles area gets is 8-10 inches per year. Over the past decades of rainfall data that I've analyzed, we received 8-10 inches of rain in 16 of them. For comparison, we received 6-8 inches of rain in 8 years, and 10-12 inches of rain in 9 years. Folks, the LA area is RIGHT ON TARGET for the rain we received this year. It's dry here, but for our native plants THIS WAS A NORMAL YEAR. Let's not try to make Los Angeles a subtropical paradise by importing water and concentrate instead on showcasing our California paradise.