Moisture sensors are the key to good conservation, but integration with the controller and web-UI is critical.
Currently, we use a Toro XtraSmart moisture sensor with our CyberRain and its really great for optimal water usage. However, to observe the status or modify settings, we're required to go into the garage. Defeats the purpose of the CyberRain.
Installation of the XtraSmart is super simple and this is very important. Before the XtraSmart (wireless, battery operated), we had an Acclima sensor. First, it was a royal pain to install the Acclima, but then the unit failed and it was an even royaller pain to find it again, dig it up, and replace it. The Acclima is a far more accurate technology and we loved the integrity of the data it provided, but in the final analysis, precision is not that important. Maintenance becomes the most important thing, and being able to install, reinstall with little effort is the overriding factor.
I am not promoting the Toro, it just happened to be the easiest choice for us at the time. Anything that meets the requirement of easy installation, maintenance, affordable, and reasonably reliable would be fine with me. Of course, it needs to be an open system that supports integration with a third party product like the rachio.
One more comment -- I see in this forum many mentions of rain sensors. I don't have experience with those, but it doesn't seem to be ideal since it only tells you how much it rained. But since you're watering the lawn you may find that you don't need to water again even if it didn't rain. This can happen if the humidity is high and its been cloudy and cool. Rainfall is an indirect measure of what you really want to know -- does the lawn need water? In addtion, there may be other sources of water a lawn gets at times like from a septic system, or shallow water table that exists at different times.