*read receipts
The device will receive the message and send the received receipt regardless of the user (unless they have airplane mode on, or the device off entirely). It won't send read receipts until the message is actually viewed though.
Personally, I turn these off completely. You'll get a response when I'm available and feel like it; I don't like people feeling like they're being ignored just because I'm busy, or not ready to reply. I also find it kind of creepy having my phone report to others what I'm doing/have done.