Looks like it has some potential but it's a bit clumsy. It's cool to be able to sign in to your XBOX profile remotely on a bigger screen but besides "cool" it's not that big of a deal unless you need to send messages to friends. The keyboard is nice if you need to search for things on Hulu and Netflix. Other than that, I can't see much of a benefit...
You can see your Halo 4 stats on it, but I don't see the point in that. Although apparently when they add Trueskill to the game, you can only see it with a SmartGlass device.
It's definitely more convenient to control the Xbox with my phone/phablet/tablet than the controller if I'm using it solely to stream. Once a year my back blows itself out and the weight difference alone would make it easier since I lose most of my ability to move for a week or two. For people that rely on XBL messaging though, its very nice, since the website tends to run slow and the app is easily accessible.