Towards the front of the case there's a ton of room. Behind the processor, the whole rear half of the case really, not so much. You can fit a thin chunk of cabling there but that's it. It's just barely enough to lay a 4 pin molex connector flat or about the thickness of an 8 wire motherboard power cable.
The problem is that the case side panels recess as they near the rear so it sucks up all the room. But you can pretty much fit anything you want in the front half, tons of room there. If you leave the 3.5" drive tower installed you would be able to do a 100 percent hidden cable install but I like the added airflow with it gone.
This morning I removed the 230mm side panel fan. It is easily the loudest fan in the computer and does not appear to be of the same quality as the 200mm fans used in the front and on the top. The computer is 25 percent more quiet with it gone. Plus, unlike the front fan, it has no filter and pulls in dust. So I removed it and now I have a trapezoidal mesh window on the side, I like it better. Case temps went up 1C and the CPU also seems to have increased in temp by 1C. Not a bad trade off at all. Things are still mighty cool and now even more quiet.
Edit: If I could have one wish it would be to have a 3.5" cut out on the front. I had to leave out my card reader. I didn't use it much but when you need one it's nice to have. I'll install it in the garage computer and read cards there if I need to.