Google Chromecast Takes a quite complicated set of things going on (control signals from phone, data signals from internet), and turns them into a seamless intuitive experience. I find the initial setup process particularly excellent. Getting a device with no user input onto a wifi network would normally be a nightmare, but the magic they do with setting up a temporary access point on the device is excellent.
I find myself never using them other than to watch youtube. What do people use it for? Why can't I buy something to just stream my screen? Like the steam link but for everything. Not just steam.
I use it for everything that my TV doesn't have a native app for, which is primarily Crunchyroll. Occasionally it's fantastic for random things -- my local soccer club FC Cincinnati was playing a preseason match yesterday that was only watchable via a livestream on the venue's website. Pulled it up on my phone, and sure enough, the little Chromecast button appeared, and it streamed without issue -- was anticipating having to hook up my laptop to the TV instead.