WHat exactly is disappointing about Chromecast? You could pay more (like double the price) and get a Roku device. What I know is that when you launch Chromecast for the very first time, it updates the firmware.
As for the lag - I agree, it can get a little disorienting, but I've noticed it depends on which device you use. When I mirror my phone screen, there's very little lag (it's not that it's not there, but it's like - maybe - 200ms delay) whereas if I were to use my Chromebook (gotten for free ), there's a half-a-second lag when I'm doing a tab casting.
Screen mirroring from the Chromebook is jittery - way more jittery than doing it from the phone - so that's why I'd rather cast the tab than the whole screen.
And the highest quality streaming you can get is 720p - they have two options within 720p as well (although the device connects to the TV on 1080p).
As for cropping the screen - that's a thing in your own TV. I've seen Samsung do it (I have a Samsung) and you have to fix the aspect ratio to have the full screen be broadcast.
AAAAaaand - they say that the casting feature is beta. If you were to cast an app, that works just as well as casting something from your phone.