Here's a list of problems the cover is causing that I've found so far:
1. The always-on display doesn't work when the keyboard is attached. Meaning you can't see the clock when the phone is in sleep mode.
2. If you start with the keyboard attached, then press the power button to put the phone to sleep mode, you'll see no always-on display. If you then detach the keyboard, the always-on display will show up, but it won't update! Meaning you'll see the same time displayed for hours afterwards (until you press the power button). Can be VERY confusing if you don't realize it.
3. When in Termux with the keyboard attached, you can see in htop that the 2 com.sec.android.inputmethod processes take about 95% and 30% of a CPU core. Also /system/bin/surfaceflinger is taking about 20%. As soon as you detach the keyboard, CPU usage goes down to reasonable near-zero levels. You can feel the phone getting a little warmer than usual, too.
4. The on-screen overlay behind the keyboard (one that receives keystrokes from it) is white! Why would they waste battery for displaying bright white which can't be seen? AMOLED is supposed to take less power when black so it's quite obvious it should be black. Displaying labels for buttons is pretty meaningless behind a hardware keyboard anyway. The bright white also casts a glow onto the rest of the screen which can be seen at night.
5. There's apparently no way to replace the stock hardware-keyboard app. It would be nice if its sources were available somewhere so you could remap / hack the buttons a bit. The euro key is pretty useless and it would serve way better as ctrl or tab, for instance. Personally I'd have it switch modes so that you could use WSAD for arrows and other letters for the rest of the important missing keys.
6. The split-screen view just doesn't work (in Marshmallow), it tells you you need to detach the keyboard first.
If not for these issues, my keyboard would probably stay attached most of the time. Because of those issues, I have to keep flipping it on and off.
Overall the keyboard's hardware is pretty nice, it feels smooth but provides the much-needed tactile feedback quite well. The fact you don't need to charge anything is awesome. The downsides are lack of backlight and the need to take up your screen space (unlike the better keyboard that the Captivate Glide had (I'll miss you, Cappy... maybe some day someone will port the latest Android to you...)).