To be clear: as of this writing, this product does not support screen mirroring or even casting from your Chrome browser if you are using a smartphone or tablet. It will only cast a limited lineup of services like Youtube and Netflix, which most smart TV's already have the ability to receive anyhow.
I was somewhat confused by other reviews, until I got this unit myself. I have a Google Nexus 7 and thought surely Chromecast would be able to at least repeat my tablet's Chrome browser (since it is after all a Google machine). Nope.
It sounds like Android support is in the works, but currently it does not exist. It was also disappointing that Vimeo is not yet supported, and Vimeo has been very nebulous on when or if this will ever happen.
I will say that when I cast Youtube on my Smart TV, the resolution is much better than the factory Youtube channel built into my TV. For some reason my TV (and my Youtube-equipped DVD player for that matter) seem to only display low-res or it takes forever to buffer hi-res. Not the case w/ the Chromecast: its been hi-def all the time with relatively quick buffering.
With all that said, I don't feel like I've gained much functionality with Chromecast over what I previously had. I'm debating on returning the item and waiting until Android screen mirroring is available, which is really what I want.