I just use a Skinomi. Thin plastic film doesn't get in the way and protects against breakage and scratches.
I've used a few of their skins. They aren't bad! I particularly like the carbon fiber ones. I previously had an ArmorSuit MilitaryShield CF skin for my iPhone 6 Plus. The cut outs on the back were flawless (no metal visible) and it covered the top/sides/bottom pretty well, but it didn't work well with some bumper cases.
My OIS went haywire on that phone and Apple replaced it, so I don't have that skin anymore. Right now I am using a cheap Skintz fake leather skin from eBay that has equally perfect cuts for the camera, mic, flash, and Apple logo, but it doesn't cover the sides at all and works with nearly every case. It looks like it is meant to curve over the edges only as far as the black glass does on the other face, giving it a "metal band" look with equal face and rear materials like the iPhone 4/4S.
I have the edges and screen protected with a Spigen bumper/screen protector (mentioned earlier) and it works great. there is not a single bit of the original aluminum exposed but it feels as thin as if it had no case on at all. There are no gaps around any of the cut-outs. My only gripe is that they didn't give you anything to protect the Apple logo. I know that it wears easily from past experience, so I perfectly aligned packing tape across the back of my phone using the antenna band as a guide before I installed the skin. That didn't work as well as I'd hoped (edges of tape hold air that makes smoothly melted distinct lines when you hit it with the hair dryer), but I did discover that the Apple logo remnants of my ArmorSuit MilitaryShield fit perfectly in the cut-outs from the Skintz skin and looked almost like the original shiny Apple logo coming through. It stayed that way for several months too, but it was sticking to packing tap (no idea how well it would have held to the exposed logo itself).
This works so well that I can't imagine wanting a case that covers the back. A skin does a better job of protecting that cosmetically and does not add thickness. It's very rare that an impact on the back will hurt your phone in the first place and the camera is protected just as well by a bumper, so I can't justify making it thicker with that kind of case. Apple used to sell bumpers back when the glass back could actually be shattered. I wonder why they stopped?