What is Reddit's opinion of 3D Compass and Magnetometer?

From 3.5 billion Reddit comments

If you can use tablets/smartphones then the "3D Compass and Magnetometer" app is totally sick, it uses the device's built-in magnetometer to show the direction and strength of the local magnetic field. I've wasted days of my life just wandering around measuring magnetic fields of things. https://play.google.com/store/apps/details?id=com.plaincode.magnetmeter

I downloaded a couple free magnetometer apps from the Play Store: https://play.google.com/store/apps/details?id=com.plaincode.magnetmeter and https://play.google.com/store/apps/details?id=spaceware.hybris.magnetometer

Both seem to indicate that the magnetometer is not really working. I'm getting something like 2000uT in absolute mode -- which seems crazy high. I'm going to keep investigating issues with the magnetometer with other applications.

Right, but post-Einstein, we should say you are accelerating! If you've got a smart phone then whip out the accelerometer (e.g. via this app on Android); it will say you're accelerating upwards at 9.8ms^(-2).

In 1900, we would have said, well, that's just a coincidence; there's no way your measuring apparatus can tell the difference between an acceleration and being stationary in a gravitational field, but it doesn't mean they are the same.

But what if, instead of being a coincidence, this is telling us something deep about the way the universe works? Albert Einstein took this thought and built it into the nucleus of general relativity. He used the equivalence principle, which says that fundamentally, there's no difference locally between being in a lift falling in a gravitational field, and drifting in deep space.

It's discussed quite nicely by Einstein in chapter XX of his layperson's book Relativity: The Special and General Theory, which you can read here.