I got this flashlight as my first 21700 flashlight to try out the new format which has higher capacity than the usual 18650, and since it is included, and rechargeable inside the light, no need to worry about another charger. I have bought Klarus lights before and the quality was always second to none, and I trust the brand.
The control is the usual Klarus dual-switch which allows flawless tactical control, and with the 4000 lumen output, this light will turn night into day easily. The duration of the High output, I think, was definitely helped by the 21700 battery as it doesn't show any obvious step-off in output, and the battery should be able to support it longer compared to the smaller 18650.
Quality is the usual Klarus, smooth anodizing, no faults on the lens and beam pattern.
The light is slightly thicker than the 18650 version, but that's to be expected and not a distraction. The UI, as expected, can be switched between Tactical and Outdoor modes. My preference is for the outdoor mode since I'm a couch commando.