I have bought a set from Amazon: 4 Mi-Light bulbs (E27, 6W), remote control, and Wi-Fi gateway. The components perform as advertised: using the remote control is reliable and immediate, lamp brightness is ok, colors are great. There's a big disappointment though: These aren't RGBW lamps! These are, to coin a phrase, HV⊕V lamps: You can have them either in color or in warm white mode, and in color mode you can only set hue and value (brightness), while in warm white mode you can only set the brightness. You cannot change the saturation and you cannot combine white and colored LEDs.
As documented elsewhere, the Wi-Fi gateway creates its own password-less Wi-Fi network to which you can connect and has a web interface with username/password admin/admin through which you can join it to your home Wi-Fi network. (Actually, my access point lets me create multiple virtual SSIDs that can be mapped to VLANs and I did so here: The Mi-Light Wi-Fi gateway is connected to a special separated network and does not have internet connectivity.) I've never used the accompanying phone app.
Instead I'm using the Wi-Fi gateway with the WifiLight module for FHEM (a home automation framework I use). This works mostly reliably, but the delay/refresh rate is awful. I'm using the same module with an LD382 Wi-Fi LED strip controller which can easily sustain one command every 150ms. With the Mi-Light gateway it's more like one command per second.
LimitlessLED (one of the names under which Mi-Light bulbs are sold) publishes a lengthy and not well organized developer page with lots of information and something they call OpenSource API. What it is though is a listing of all the commands that you can send to the Wi-Fi gateway, and that's already a big help. Besides the FHEM module mentioned above, there already is a lot of code in all the programming languages of the world (including PHP und bash) to send commands to the gateway.
In the openhab (which is another home automation framework) Google Group there is a thread from someone who has opened their bridge, discovered that it consists essentially of two boards (a Wi-Fi to UART side, and an UART to proprietary 2.4GHz side) and replaced the Wi-Fi connection with a wired LAN connection for better reliability. This thread contains a lot of information: It lists the 2.4GHz chip for the proprietary RF system as a PL1167 and even has data sheets for the PL1167. The protocol on the UART connection is simply the same protocol as on the Wi-Fi: the Wi-Fi→UART converter is a dumb standard component that is not Mi-Light specific.
Some of this information was previously collected and linked to from https://wikidevi.com/wiki/MiLight, there's also a link to a Mi-Fi bulb teardown.