I agree that CC implementation in XBMC would be awesome, but I've been using a sort of "integrated" solution to cast my (temporary) XBMC library.
I just moved and haven't set up all of my AV stuff or my desktop computer (just using a laptop for now). I picked up a Chromecast to watch... stuff (Netflix) on my TV while I redo my fairly complex setup including my main XBMC library. I'm really impressed with the device, IMO much better than watching Netflix in a browser window or XBMCflix. Anyways, I've had the Mediabrowser3 server, which is fantastic for scraping metadata / artwork for media and integrates with XBMC via the MB3 plugin, installed on my desktop since around Gotham. Also the Android app is quite nice, and as I've learned, recently integrated Chromecast streaming (!!!). I've always thought Plex was ok, but I have several problems with it. The show-stopping one is that the server simply will not connect through my vpn, and as my research has revealed, there's just no workaround. MB3 server (on LAN at least) isn't affected at all by connection / disconnection to the vpn, so that alone makes it the clear winner.
So while I've mainly been using the CC for Netflix, I've also used it to stream new content from my laptop, on which I installed the Mediabrowser server and organized new content XBMC library-style (which I'll add to my main library once it's up and running).
TL;DR Plex kind of sucks, IMO. MB3 is awesome and integrates with XBMC. MB3 can then play any content from your XBMC library to your Chromecast via the Android app (to a different, or even remote, TV perhaps?)