It's Not a Roku
The good news for Roku is that the Chromecast is not a Roku replacement. I suppose it could be; but, both devices would serve very different purposes in my household. I thought the Chromecast would act like the Roku where you'd have a billion channels of content from which to browse. The Roku is a media aggregator of sorts, a platform for discovering and viewing video and audio. I heard it does games too.
What It Is
The Chromecast, on the other hand, is nothing of the sort. It is merely the screen you can use to display content from other devices. Chromecast is not a platform in itself. The platform is your computer, your laptop, your tablet, or your phone. Your personal device acts as the aggregator. The Chromecast is only a venue for your content to show.
I can see the Chromecast as something that a household or even a business must have, even if it is not the principal media gateway. I can certainly see people continuing to subscribe to cable, satellite, or other services available on the Roku and other similar boxes. The Chromecast would be an addition to those.
The difference is that the Chromecast is a very good shared resource. You can have multiple people with different accounts use the same device to view their content. The concept is flipped from "one to many" to "many to one".
Why This is Great
The Chromecast is great, even with its currently limited apps, because it allows you to share your TV with others without having to get into the business of user names and passwords, or worse, messing with your entertainment cabling. As long as you share your WiFi access, guests can project their content on your TV.
This is very useful in business settings too. It reduces the whole mess of having to carry a VGA, DVI, or HDMI cable to make a presentation. Wait, let me back up. You can't just display anything on a TV. Currently, you still need a Chrome browser with the Google Cast extension to be able to share a browser tab. You are also limited to Youtube videos and some other streaming video/audio services. That means your PowerPoint presentation would have to be a Google Slides presentation through a browser tab. Similarly, you would do the same with spreadsheets and text documents.
Just Pick A Screen
The upshot of all this is that you could just pick a screen to view or show your content, and that's it. No need to hunt for cables or fuss with bringing your media box over, or hunting for the media again on an already connect device. Pick a screen and view.
What I Would Love
Our home only has a couple TVs. One in our living room and one in my office, which is being used as a computer monitor. If our household were the sort where everybody has a TV in their bedroom, and TVs in the kitchen and other living areas, I would love each one to have a Chromecast.
At the very least, I would gift one to the people I most often visit. I'm sure you've been in the position where you visit somebody and they invariably tell you to make yourself at home. So, as you settle in with the TV, you can't figure out the remotes. With the Chromecast, problem solved. You only need to turn on the TV, switch to HDMI channel, and connect your phone to the Chromecast to watch what you want to watch. I'm assuming you've already asked for the WiFi password.
If everybody you like has a Chromecast, you would have access to screens everywhere you go. I don't see it as a main viewing device. I see it as an auxiliary viewing device where anybody can access without hunting for cables and connectors.
I get it now.