Depending on whether the app's media type and location is supported by Chromecast, the 'source' device may actually be doing the work of fetching and rendering the material and 'casting' the A/V output to Chromecast, but commonly it's just sending the URL and various tokens and chromecast is doing the actual fetching/decoding.
It's similar to having an Airplay-only device on your TV; bearing in mind that they are similar protocols but not the same nor interchangeable. Its main disadvantage is that it can't play content that's local to your network, so if you have your own movies and things you have to play them on your device and screencast it to chromecast. Which may or may not be well supported and look decent. For several good reasons I'd rather tell the TV-attached gizmo "play this file, which you can find over on that computer", than tie up some other device playing it and throwing the video to the TV. You can do the latter with Apple TV as well, but the thing is you don't have to, at least for any content that's supported by iTunes.
I ended up buying a matched soundbar from the same vendor (Panasonic Viera), one that uses HDMI and connects to the ARC-enabled port on the TV, which means basically all three gizmos (HTPC+CEC running XBMC, TV and soundbar) can be controlled with just the TV remote. It also means if I turn off the TV and just use the HTPC/XBMC + soundbar for music, the xbmc mobile app can control the speaker volume.
It's all basically as straightforward and usable as it can get. If I were going to buy a Smart TV again I might get a Samsung or Vizio, as there's a Plex app available for both. Which is a whole nother topic. (I don't use Plex myself but it's what I'd recommend to pretty much anyone I didn't recommend Apple TV to, i.e. someone who doesn't have a houseful of predominantly Apple goodies already.)