DirecTV's app is only "really good" if you are home, on your own network, and your receiver is also on the network. Then you can watch probably 60ish channels on your phone/tablet, plus any recordings on your DVR.
If you are off your network, the app will let you watch maybe about a dozen channels, and no DVR recordings.
Anyhow, for watching the Olympics, I'll be using the NBC Sports Live Extra app. I believe they are supposed to be streaming every single event via the app. As always, you need to be a cable/satellite subscriber in order to stream it... but it seems to work very well.
As long as I can get my 4G signal.