I think the history of the flat-sided iPhones is a mixed one at this point. The first one, the iPhone 4 series, I still consider one of their greatest phones. Smaller of course, but also thicker, which mitigated the flat sides. Great ratio of weight to thickness to sides.
The 5 series I liked less. It got thinner, and that to me made the (uncased) sides a little less comfortable. First world problem.
But the 5c got it dead on. Flat sides with the rounded corners. "Unapologetically plastic." A joy to hold uncased, and a little bit heavier than the stage-prop-light 5S. Perfect marriage of holdability, comfort, and grippiness. And I was a fan of the colors. Gosh I miss the 5c.
Then when the 6 came out, I felt it was a drop waiting to happen. No flat sides to solidify the grip, at the time the matte aluminum I think. And of course the camera bulge...I don't think I'm returning to caseless carry again. That makes me sad. Because of the camera bump, even now that it has gone back to flat sides, it won't matter because I'd have a case.