You know, I'm glad I read this thread because I have a RPI just sitting around here collecting dust and my father (recently moved in with me) likes my Roku 3 way too much and he only watches movies I have via Plex on the Roku 3. I'd like to get my Roku 3 back into my bedroom and give him the RPI. Eh, after thinking about it I think he would just buy a Roku 2.
And as an alternative to the RPI, if you want an all inclusive unit and I do believe a smart phone can control it if you don't like the included simple remote, the Roku 2 or 3 is a very nice option and I have no problems using it to watch Plex, Netflix, etc... It's a dedicated device. There is a new Roku 4 coming out which I understand supports 4K TVs but I have not seen it myself yet. Oh, there is a Roku Stick as well, similar to Chromcast but comes with a real remote control.