Late last week there were many reports online that the iPhone 5s has serious problems with its motion sensors: the accelerometer, gyroscope, and compass. Revealing photos, such as the one that accompanies the report by CNET, give clear evidence of the problem. The CNET photo shows an iPhone 5 on a table next to an iPhone 5s, each running the built-in iOS compass. The phones should show the same reading, but there is a large discrepancy. Similarly, the accelerometer is also error-prone. Gizmodo tested its accuracy by using a racing game that relies on tilting the device to control the action. They placed the iPhone 5s on a level surface and didn't move it, yet the device immediately showed a leftward tilt and sent the car in that direction. For hardcore gamers this is a major issue.
The inclinometer also gives evidence of the problem. Gizmodo compared the reading from an iPhone 5s to that of a standard bubble level and found a discrepancy of 2–3 degrees. They say that testing by others has shown a discrepancy as high as 4–6 degrees. Their testing also shows a problem with the gyroscope that's about the same magnitude.
It's not clear how many users are affected, but it appears to be widespread. Apple hasn't yet acknowledged the problem. Some iPhone 5s owners have returned their defective phone for a new one, but ended up with the same defect. Others no longer had the problem after getting a replacement phone.
It's not known whether it's a hardware or software problem, but given that iPhone 5 users who have upgraded to iOS 7 aren't having any problems, it would seem to be hardware-related. If so, that can't be easily fixed with some sort of software update or a firmware fix. The problem seems to lie with the new M7 motion co-processor chip, which was highly touted during the iPhone 5s launch.