Roberto Mateu spent a week without a case on his iPhone 6s:

I have to say… while still slippery as hell, forcing myself to use the phone without a case is very nice. It’s so much easier to get in and out of my pocket, and feels much more sleek in the hand. I also enjoy being able to reach the screen borders for easier gestures. It has made me use the 3D Touch of the left corner to switch apps much more natural than before.

I’ve used various models of iPhone since the day of the original’s launch in 2007. I have never used a case on any of them and have never regretted it. In my honest opinion, it’s the only way to go.