I have an old xbox modded to use XBMC. Granted I have not used it in a while but from what I remember it was a cool program. I am sure it is even better now.
ive used xbmc since first release and now on my htpc, its better than ever. With a little bit of effort and the use of some plugins you can get your self one sexy looking media centre and there is loads of help on their forums in doing so! Ive also tried plex but for me xbmc still cuts it
Do yourself a favour mate and grab yourself a self contained XBMC Linux distro called OpenElec, this way you boot straight into XBMC when starting your HTPC.
And here is the link. http://openelec.tv/
You're welcome.
I use Plex for about half a year now, and it's really cool. I had Plex and Plex media server running on my HTPC (is going to be just a home server as our new Samsung Smart TV can access the media server over W-Lan and the Plex app from the samsung store)
It works great with the TV, PCs and Macs and there's also an app for iOS (not sure about android)
I've used both Plex and XBMC, I prefer the latter. Plex is nice if you want something that works right out of the box, but if you like customization, XBMC blows right past it.