When navigating through the UI of a console and actually playing games, the addition of a screen to a controller can definitely be a plus. It's all about the right implementation. I can't help but to laugh when people classify it as a gimmick. I wonder what Sony fans we're saying back when Nintendo first added an analog stick to a controller, let me guess, that it was a gimmick ?
The only negative I see is that it drives cost up. I wouldn't mind paying 50-70$ more for a PS4 if it came with a controller similar to the Gamepad. As long as they also keep the regular DualShock available for purchase, just like Nintendo is doing by offering the Pro controller.
Im not saying it would be the smart marketing choice by Sony, but as gamer, that's what I would of like from them.