Well I got my Xiaomi Yi from Amazon in 2 days. Pretty cool little camera. This post will be edited as I find out more about it and use it in the next months.
My Camera Version: YDXJ_v22 (aka z22)
My Current Firmware: v1.2.12 (Stock/Shipping firmware was v1.0.2)
My current SD-Card: Lexar 32gb Micro-SDHC Class-10 UHS-1 (300x-45MB/s)
While I could have used my (mainly DroidPlanner) Android tablet (and what I suspect is a more mature App), I carry an iPhone-5, so I downloaded it's newer iOS App version: v1.12 from the App-Store. All my usage notes (for now) will be with this app.
I had to connect to the Yi's WiFi via Settings like a conventional HotSpot. I ignored the Yi firmware update a couple of times, but on the third run or so it just decided to do it anyway. It did it over WiFi. At the end (after the Yi turns itself off) you turn it back on manually and it's done. Anyway, that's how it's firmware got updated so soon. The App didn't update, but the Yi now has more options.
Took some videos and pics and it works fine so far. Battery seems to last about an hour, but there is a lot going on inside this little camera. From completely off, it takes a few seconds to boot-up. Would have been nice to get a lens-protector.
At the time I ordered from Amazon (Sept. 2015), I got the z22 version. Mine seems to work fine. Not sure if there is much of a difference, but they also have the new Xiaomi Yi z23L version . I doubt there is much of a difference (likely just the latest revision). There is also now an official USA Edition (so get that one if in USA).
So, you can use the camera's WiFi while on the ground, but be sure to turn it off before taking off. Both 2.4ghz RC (main Radio-Control) and WiFi/Bluetooth operate on similar 2.4ghz frequencies. They will generally interfere with each other, as this video demonstrates. In this video, notice how poor the range and thru-put gets (laggy) as both fight to use the same 2.4ghz band.
Battery last for 60 minutes (recording at 1080p@30fps) at that video fits on 32gb SD-Card.
In May 2017, iOS App (on iPhone-5s) is updated and still works fine. Downloaded latest camera firmware.