Having had this on Android for years, I have yet to really use this feature. I never know exactly what Android considers a poor signal in relation to my mobile network signal. Given the iPhone's tendency to disconnect from WiFi in an idle state already, I might wonder how this feature will pan out in the long run. One might think that because a home WiFi network would be ideally stronger than a mobile network's signal, given how signal levels fluctuate on both wifi and mobile network, this might be something to keep an eye on. Who knows what MIGHT be downloading or updating when a phone decides which is stronger, and locks onto that signal until the process is complete, and in turn mobile data usage increases.
I Am waiting to see how this works. The iPhone 6 certainly hangs on to wifi far longer than it has a useful signal. I'm sure we have some that think they are on wifi and get switched to cellular and run up the data.