Update, 10/17/2005:
Some new photos here:
5) Added two 2500 gallon black polyethylene tanks, from local farm supply store, about $1100 each.
6) Transfer pump, about $30 from Harbor Freight.

And some comments:
1) As we faced a long hot summer, I became apprehensive about our storage volume; hence the two additional tanks.
2) After I placed and hooked up the new tanks, I decided I needed the transfer pump because the new tanks are a different height than the original tank.
3) We started metering water 2/6/2005; as of 8/17/2005, we have pumped 16,740 gallons. That would be about 190 days or just under 100 gallons per day. Our normal use is 40-50 gallons per day plus about 100 gallons for laundry about once a week. We lost an estimated 2000 gallons through an un-glued pipe joint and intentionally put 400-600 gallons on a newly planted shrub.
4) This past summer has been dryer than "normal". I estimate that we've only had about 15", February through September. Our "normal" annual rainfall is 30+".
5) Most of the 16K gallons has been filtered. The pressure drop across the filter remains about 5psi and the element has not been changed.
6) We filled the two "backup" tanks with the transfer pump and have used none of that backup supply. Lowest level in the main tank has been about 2', or about 25%.
7) When Hurricane Rita threatened us in September, we were considering how we might handle several days with no electric power. Our drinking water supply is assured through the gravity feed spiggot near the main tank.
8) The gutter system does not collect all the water during a heavy rain; the gutters spill. With the generous roof area, that is not a problem; if I were to do it again, I would still install only four downspouts due to labor and other costs. 1" of rain does seem to give us about 2,000 gallons of water.
9) The system has proven itself well enough that we have suspended our "city" water supply. Even though we used none, we have been paying ~$12/month to have that supply available to us. Now, let's see how long it takes that $12/month savings to pay for our $9K-$10K rainwater system :-)
9) The 1/2" "always open" drain valve, that is supposed to prevent long term water presence in the downspouts, clogged. The drain pipe is bushed down from 4" to 1"; I replaced the 1/2" valve with a 1" valve. If I had it to do over again, I would make the drain 2" with a (removeable) 1" valve.
10) I am still looking for some sort of a quick patch to have on hand in case of a punctured tank.

Update 11/5/2006
My Grundfos pump died and I replaced it with one of two that I had bought on eBay as spares. I had expected the pump to last ten years or so. If they run less than two years, that will really run my average monthly expense up. Water meter reading: 43540

Update 6/12/2007
59,140 gallons on the meter. Still haven't changed the 5 micron filter. Let's see...
1) We've been metering water 28 months, so our consumption is averaging over 2,000 gallons a month. That's more than I expected; I thought we might be using less than 1000 gallons per month. As I've mentioned, we've lost some of that water to leaks.
2) In our ~35"/year area, we should have had the opportunity to capture about 160,000 gallons of water over those 28 months. We are using a larger fraction of the available water than I imagined; we're using over 1/3. I would have guessed that we were using only about 1/10 of the available water.
3) The least volume of water we have had in storage over those 28 months is about 3000 gallons out of our total capacity of about 11,000 gallons.
All in all, we are very pleased with our water supply. Very high quality water, especially compared to the community supply. Our water fixtures no longer collect hard water deposits and I presume that our hot water heater will last longer. We have been without water about three times, due the the pump failure and leaks. That is no less reliable than the community supply, but when the community supply went down, all we had to do was wait. Still, the community water is almost certainly cheaper; I have about $11K plus labor in our rainwater system. I will never be able to pay for it with the ~$400 / year saved on the community water cost.

Texas Water Development Board rainwater site

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