I did look at the Chromecast but figured I wouldn't be able to run Kodi on it. I hadn't heard of the Roku before, I might have a look at one sometime.
There are Kodi and Plex apps for the Roku. As well as apps for just about every media company out there, and several aggregators for media. The only app I haven't found is one for Google Play Music; but I get around that by using Miracast on a tablet. 95% of our media viewing happens via Netflix and Plex, which run beautifully on the Roku devices (we have an Express+ and a Premiere+).
You don't "run" anything on the Chromecast. You find the content you want to play on your phone or tablet, then press the "Cast" icon and it sends the URL for the media to the Chromecast, which then starts to play it. It's entirely dependent on a separate device for controlling playback and selecting media to play. Some people find that awesome; others (like me) find that to be horrible.
Oh, one other major feature that seemed to be missing on a lot of players is support for HDMI-CEC. If there's no good CEC support my AVR won't automatically switch to the correct input channel, which means I have to juggle 3 remotes (one for the player, one for my AVR and one for the TV)

Yeah, I know about the Logitech Harmony and similar products but nothing beats good CEC support.
The Roku Premiere+ supports CEC and turns on our TV. I think it used to control volume as well, but I changed something on the TV and now CEC doesn't work anymore.

There's also a Universal Remote add-on for the Roku remote that allows it to control more aspects of the TV.
The biggest feature and greatest weakness of the Roku Premiere/Ultra is that it uses a wireless remote (meaning, using 802.11 Wifi). This is great because you can be essentially anywhere in the house and control the Roku. And it supports audio redirect to a headphone jack on the remote for private listening (an awesome feature for late-night TV viewing). But it means you are pretty much forced to use the Roku remote along with a universal remote for controlling the rest of the entertainment systems. AFAIK, there's no Harmony support for a Roku.
For us, with just a simple TV output to a 2.1 stereo system via stereo RCA cables, the Roku is a great system and the wireless remote is godsend (the Express+ upstairs has an IR remote and it annoys me to hell using it). For someone with a more complex audio setup, the wireless remote may be a hindrance.
I used to use an HTPC for media with all kinds of fancy applications installed, originally on Windows XP. Then migrated to Plex on our FreeBSD server and found ourselves using just a Chrome web browser to access the Plex web client, Netflix, and a few news sites. Migrated the HTPC to a smaller build running Linux and Chrome, and found ourselves using even fewer tabs. But maintaining them was getting to be a chore; it wasn't as set-it-and-forget-it as I wanted it to be. So we picked up a couple Roku boxes and now I don't have to manage anything but the Plex server.

And even that is fully automated now, so I don't have to fiddle with anything at home except the remote.
