I have a chromecast running hulu, netflix and hbo go. The thing that annoys me is that because of unotelly, for some reason I have to first start programs on the phone and then enable chromecast, it's not really seamless but I guess it's just knit picking on my part. Youtube however doesn't ever have this problem, and i'm sure plex won't either.
Currently i'm running an openelec htpc and a rasperry pi. The thing I now have to consider is if I move to the chromecast/plex setup i'll have to get a stronger server as I only have a Hp microserver N36l. Will plex handle hd content well enough on the chromecast, and if so how big would the server need to be to transcode 2 full hd streams. To me plex seems to be more feature rich and development seems to be moving faster then xbmc, my only gripe is running an i7/xeon server seems a bit overkill.
With regards to the chromecast, i'm sure we'll still see big things to come from google and the android community, I'd buy it just for the sake of being able to fiddle with all the new crap.