I'll have had the Mi Box for two weeks tomorrow.
I like it, but at least mine has a bug where it will sometimes default back to 720p if I ever restart the system. But otherwise it'll stay at UHD/60. So not a huge deal.
Never having had Chromecast, I do appreciate the Mi Box's ability to Google Cast. Works great for using Slacker which doesn't have an Android TV app. Oh... And disabling the XiaomiLeanbackCustomizer app removes the section of pre installed Mi Box recommendations, giving more of a stock android tv look. Moreover, I Like the power button on the remote to put into sleep/standby. Though, the lack of play/pause on the remote is annoying.
Things I'm a little questionable on are the scenes with black backgrounds. YouTube, mostly. Just seems like maybe they are more blotchy and pixelated with Mi Box than say, similar videos watched on my TV's built in YouTube app. But I've not bothered to actually compare quality between the two, so Mi Box picture quality might not actually be worse.
And wifi I suspect is faster in my TV (stronger signal maybe) than with Mi Box, because I think maybe 4K demos from YouTube probably look a little more impressive on my Samsung TVs built in YouTube app.
Been thinking about just setting the box to 1080p. I'm Wondering if overall performance might slightly improve as a result??? I dunno if that's likely. But I'm pretty sure I don't need 4K material on my nowadays relatively tiny 55" TV, even if it is technically 4K.
That said, I'm happy with Mi Box for the simple stuff I do---YouTube, Netflix, Vudu, Google Music, etc. Can't go too wrong. After a $5 vudu movie credit, the price was only $64.