I own a Chromecast and am just waiting for Black Friday or Winter Deals to get a new one for my other TV. I used to watch Netflix and Hulu in my TV's native apps, but compared to touch input on a phone/tablet and most importantly, keyboard input when searching, using the Chromecast instead became sooooo much easier. Granted, not all apps support Chromecasting, but for those, you can still use screen mirroring.
Also, the fact that you can be using your computer and cast any Chrome tab or just be watching a video (if the site supports it) and cast the video to the TV while keeping the computer 'free' to do other things, is just awesome. Just last night I was trying to watch the debate online and since the CNN app doesn't support Chromecast, I turned to the website and voila! The video there did support it, so I just 'casted' it to my TV, and kept on working in the computer.
Another cool use is that while you're watching a video or pictures or whatever you're using on Chromecast, your device is free to keep doing whatever else you want, including trying to find something else to watch without having to exit to an app menu. Just find it on your phone/tablet, select CAST, done. I no longer endure Hulu's auto-suggestions at the end of an episode, I just select what I want to watch during credits.
I only wish the Chromecast would be 'detected' even if you're not on WiFi. My phone, for instance, has unlimited data and I hardly turn WiFi on at all. But to Chromecast something, I have to connect it to WiFi, even though the actual video/media streaming is being handled by the Chromecast itself, not my phone.