I have the Ringke Slim for my 6. I love it
My first case for the 6 was the Spigen Slim Armor, and while it was a nice case, i wasn't a fan of the cutout, the buttons were a little iffy, and it was rather slippery, so much so that it fell out of my jacket pocket when getting out of my car
The Slim was a nice trade up, first off, it lives up to it's name, it's slim. Feels great in the hand. Yes, the back is slippery, but the TPU sides offset that by offering great grip, doesn't feel like the phone is going to slip out of my hand or pocket. The port protectors are a nice little added bonus too