I looked at the hydrawise this afternoon. It looks like indeed it can have by-zone control of 24 zones (up to 36).
It also has a different business model than the Rachio. Hydrawise charges at least $60/year if you want a weather feed from anyplace other than an airport. I would think, thus, that most people will have to digest $60/year in addition to purchase price - airport data just is not local enough if you want to put your controller on full automatic mode.
So I think the summary is, for those needing more than 16 zones, there's indeed a solution on the marketplace today. But it's going to cost you.
I measure the amount of time each zone is on. Here's an example ( http://www.welserver.com/perl/plot/WEL0343/Sprinkler1.png ) showing the run time of zone 10 and all zones collectively. I monitor each zone wire's voltage and bring it into a energy monitoring unit ( http://www.welserver.com ). And I have the monitoring unit configured to produce the example chart (along with the other two sprinkler related charts).
Via manual measurements at the water meter, I do know each zone's average GPM. Via further configuration of the monitoring unit, I could easly translate time the zone is ON to gallons used. And if I wanted to do this for all zones and aggregate for the sprinkler system. But, for me, I'm good with simply the time durations.
My monitoring unit is an Internet Appliance, and thus does issue text messages or emails or both. I have it configured to send both for any zone that's on more than one hour total during any given day. The Rachio has been reliable to the point that I haven't ever had a stuck valve, and thus have not ever received the alarm.
Parenthetically, I'm a little uncomfortable stringing a cable from my sprinkler controller out to a water flow measuring device near the water meter. This cable will serve as an antenna for surges coming into the Rachio. I think I'll wait for a wireless solution that transmits the flow information to the Rachio.