Simplicity and its loyal companion reliability beat unnecessary complexity any time.

Why depend on something that needs battery power, functioning electronics, a signal, and possibly a subscription merely to act as a compass. Besides that, how much space does a key chain style compass actually consume? The small size and weight of this type of compass also promote having a backup system.

From my experience, Sun makes good small compasses, as does Tru-Nord.

Here's a useful test to perform: Submerge in water for a couple of minutes the Marble compass and the appropriately setup cell phone and then report back which one works better as a compass. Works is the operative word.