Has anyone used this detail of weather information to improve gardening at all?
Absolutely. Davis systems are marketed to farmers, nurseries and other commercial businesses, and are used to determine soil moisture and how much supplemental water is needed. Given that fuel costs are among the highest costs a farmer can incur, irrigation can be quite expensive especially in a drought year.
One of the nice things I like about Davis is the wide variety of sensors. They offer soil moisture sensors so you can tell precisely when to water, and soil temperature sensors for knowing when to plant. Even without the soil sensors, if you have a solar sensor one of the values a Davis station will compute is "evapotranspiration", an estimate as to how much water is in your soil based on wind, air pressure, temperature, etc.
A Davis console can also be interfaced to a set of "alarm" relays. You can set certain parameters that will operate equipment (rain and soil moisture levels could control your sprinklers, inside temperature could sound an alarm if the temperature dropped to the point where your pipes are about to burst, etc.) It's quite the versatile environment monitor.
The drawback to a Davis system is, of course, the cost. You can spend anywhere from $495 for their low-end solution to thousands after adding transmitters, sensors, loggers, etc. I wasn't sure about the cost at first, but I bought the wireless pro and I've been amazed at how often I refer to it, and how much my wife likes having it, too.
And sure, we could have gone with an Oregon Scientific or La Crosse station, but after reading various forums about peoples experiences, I was sold on a Davis station.
Of course if you're only looking for a way to shut off your sprinklers on rainy, cold or windy days, Hunter, Orbit, Toro and other manufacturers make shutoff sensors designed specifically to control your sprinkler system. They're convenient to install and usually much less visible in your yard than a full weather station, but they're just on/off switches and don't provide you with readings.