That 'cast' functionality has improved and enhanced my life more than I expected. It saves significant time, hassle and stress to be able to connect to either of the two main screens in my house from my laptop when I want to show someone what's on my screen (remote desktop in reverse is how I think of it). No redundant searching, or going to the trouble of sending someone a link. Just cast and it's there.
However, it works best for images and text-- video or graphics tend to have more issues, and there is definitely a lot of lag, as with remote desktop.
Some web-apps, like youtube, do the casting for video in a more complicated manner.
Rather than actually doing the 'remote desktop in reverse' they essentially instruct the Smart-TV to go to the same video end-point and start streaming. Thus, none of the rendering, decoding or even downloading will be done on your laptop.
This is slightly worse from a control perspective, but quality wise it is great.
However, it works best for images and text-- video or graphics tend to have more issues, and there is definitely a lot of lag, as with remote desktop.