Yeah, Plex was originally a fork from XBMC, but it already has a Roku channel, and Andoid and iPhone/iPad client apps that work with it while supporting OS X, Windows, and Linux on the server side. Plus, you can automatically download the movie/tv synopsises from free internet databases, so it ends up looking like your own personal Netflix.
The only downside I've encountered is that I encode in Divx, which Roku doesn't support natively, so I can't do a direct stream to Roku, it has to transcode it. On my Android tablet, using my my AMD E-350 as the server and direct streaming, it shows virtually no CPU utilization, but if I have to transcode, both cores on the E-350 max out. On my i5-2500, CPU utilization is low when transcoding.