I'm returning mine for a couple of reasons. Video playback is not smooth. There are dropped frames everywhere I look. WatchESPN and the Vudu app are particularly noticeable. If you watch the scroll on ESPN, there are stutters -- not as bad as I've seen, but not smooth. This is my primary test of a media device. Streaming quality should be indistinguishable from satellite/cable or even better quality given how much compression happens with satellite/cable.
The other problem is video quality. The picture is noticeably washed out compared to a 2nd-generation Fire TV, Apple TV 4 and the Nexus Player. Yes, the first two devices cost more, but the Nexus Player was less by the time I bought it. The Nexus Player is slightly slower in its interface, but video quality is miles above the Mi Box.
I'm skeptical they can fix these problems with firmware. The hardware just isn't as good as other devices. I'll stick with my Nexus Player. The Mi Box may be worth it if you absolutely have to have 4K, but based on reviews, it looks like there are issues there, too.
You may want to hold off on returning for a week or so. See if they have further updates.