All right, I shot one. My LGS has a huge collection of curiosities and I finally managed to convince the owner to let me shoot his KSG to dispel some of the myths.
The gun is perfectly balanced, will hold FOURTEEN +1 shells of whatever you want, and you can load either tube with what you want and switch between them as you please. This seems like a great idea in theory, but in emergency situations I'm guessing you'll just be racking the forend and hoping the front part of the gun goes bang.
Is it reliable? Maybe. It ejects down, if you keep its poo chute unobstructed with your forearm it won't jam. It requires a smooth and forceful action to get the forend going to chamber another shell. Just like you can jam an 870 by not being smooth and forceful with the forend, this how you can mess up the KSG. Rack it like you mean it in a quick and aggressive motion, and it will be fine. When it jams, tho, oh man it gets jammed.
Switching ammo on the fly? Just hit the selector switch between the magazine tubes. There is sharp stuff in there, and you may pull back a bloody hand.
My friend claims he has fed his KSG over 1000 shells and has not had it fail, I tried a variety of ammo in it but wasn't able to match his count due to financial considerations. It's seems well made and if I can find one for 900 bucks I still may get one just for the curiosity of it.
My only critique is that I have been shooting the 870 in some form since I've been 12. I could strip that gun down to the bare receiver without ever reading the instructions as a kid, and it's so modular that you can modify/replace everything to make your own personal gun for whatever situation you need. I typically convert my 870 between skeet shooting and home defense, it takes less than 10 minutes to do so. The receiver has over 20k rounds through it and I have several barrels, forends, and stocks. It has never malfunctioned except due to user error.
The KSG is a new animal and hopefully will lead to a modern redesign of the shotgun, the 870 is just so perfect.
