For me (AT&T Moto X), Moto Assist works great. I only have sleeping mode and driving mode enabled. Driving mode appears to be based on the location sensors. Assuming you have the location settings enabled (no tin foil hat here), the device seems to go to driving mode whenever it detects you traveling fast enough to be in a car. Walking / Jogging do not trigger it.
Whenever I am driving, the Moto X is connected to my car's Bluetooth system, and when I receive a text message, it says: "<contact> sent you a message. Say "listen" to listen to the message." You say "listen" and it says "<contact> said <message>."
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