This iPhone 5 s has gotten a lot of attention for it's camera (which has some excellent features) but I think the M7 coprocessor has some interesting potential. It's been suggested that the M7's purpose is to save battery, but this isn't really the case. The M7's function is to passively monitor the phone's motion sensors; that is the accelerometers, gyroscope etc, continuously. This process does use less battery than the phone actively monitoring the motion sensors; but previous iPhones could only monitor motion when an app was specifically doing so. In this sense 'active' means to utilise the A7 'System on a Chip' processor which is significantly more power hungry and undertakes all other computing legwork for the device. It's true therefore that however efficient the M7 is, it's inclusion will increase power usage simply because it is performing a task that was not being done on previous phones.
The benefit is that when an app does become active, it can access the whole day's accumulated motion data, allowing for passive data collection over much longer periods of time at significantly less battery usage than actively monitoring for that same period. The M7 therefore radically improves the motion data collection functionality of the device (removing the benefit of external pedometers and the like). This is great but currently has limited application: sports performance, exercise tracking, even traffic conditions. Clearly useful, but not necessarily useful to everyone.
It seems to me though that this data collection function could be expanded upon, though it may require future revisions of the chip, to include other kinds of sensor. Consider that heart rate and a number of other biometric and health measurements could be captured by a device on your wrist and passively collected by an embedded M7 (or future M8 or whatever). Such integration into a smart watch would not engage the watch's screen or primary processor allowing for a lot of data to be collected for only trivial battery usage. Your phone can then access this data when it is relevant, transferring a large amount of information all at once and again preserving battery life.
I think this kind of coprocessor is something to watch. As our personal technology focuses more and more on capturing and processing personal information being able to do so continually and passively is necessary to form a complete picture of our activities and our health.