I bought the official
cyan flip cover a couple weeks ago for my mum's new Lumia 640 XL and thought I'd post some pics.
It's very solid with a nice matte finish. The cover is leatherette with the Microsoft logo embossed at the bottom.
There are two card slots on the inside of the felt-lined cover, which helps to keep the screen clean. Pro tip: don't regularly touch or handle the inner cover as it goes grey and loses its smooth finish (much like on my 640's flip cover).
The back is extra thick to protect the device from falls, however the sides remain the same thickness as the standard shell.
The camera hump is all but eliminated too.
All-in-all, it's an excellent cover. Pick one up if you can!