I've had that Xbox Smartglass app for my phone for a couple months, but I just really started using it a bunch in the last couple of weeks. You get all your Xbox live messages on it, and can use your phone for messaging back. It's super handy for using it to communicate with someone who doesn't use a headset, or can read and type English, but not speak it very well. It's also nice, you don't have to back out of the game, to check your messages. You can just leave that message screen open on your phone, for communicating in the lobby, about which race or car class etc. you don't have to even have your phone on wifi, or connected to your Xbox for it to get messages. I think you can even use it to receive and send messages to Xbox live, while your Xb1 is turned off, or you are away from it. If you connect your phone/iPad to your xb1 using wi-fi, you can use your device as a remote while watching movies.
I'm sure most of you know this, but if not I figured I'd share this information. The messaging part really impressed me, it beats using a controller to message.