Plex 99% of the time, YouTube the other 1%.
In my mind, it is missing a few key features that the Apple TV has.
1. Display mirroring
2. Just about any video you pull up on an iDevice in any web browser can be streamed to an Apple TV (universal support), and at this point almost none can be streamed to Chromecast from an Android device.
3. An Apple TV doesn't require an extra device to be functional
But the Chromecast is a good Plex player for the price. Other than that, its not that functional. But I guess that makes sense considering its only $35.