Plex is the only one worth using.
I still use XBMC, but it won't work with the Chromecast. You could already stream from plex to your chromecast, if you were a premium user.
If you are going to use a plex server though, you may as well use NOW TV boxes as your receiver. They are a tenner each. Which is 1/3 of the price of the chromecast. I know a lot of people that use plex to share all their media to their friends. They just buy a NOWTV box for a tenner and have instant access to 1000s of films and TV shows.