Mine arrived today ...
When I tested the runtime I got about 4.5 hours.
It gets a bit warm. While I did the runtime test, I was also testing one of the DE `900 (really 400) lumen' lights, the two mode one in low light mode, and the other light was still cool hours later, but the red one was rather warm. Not hot, but warm.
(Side note -- the 2 mode 400 lumen light doesn't do PWM to adjust the brightness like the 5 mode one. Instead, it puts about 7 ohms of resistance in the circuit in dim mode, and it doesn't seem to even get warm, but still puts out a lot of light.)
Like the other lights of this type, the battery fits a bit loosely, and so you'll want to put a bit of a shim, perhaps part of a business card, in there to make it fit more snugly and vibrate less.
All in all, the red light is certainly bright enough, though I'd prefer a bit more efficiency to get longer run times. I'd prefer the beam be a bit less focused for a rear light -- a concave lens might be the way to do that. I'd probably want to grind off the `tactical' ridges near the glass to increase the possible viewing angle a bit. Should make a nice main rear light, but I'll make sure I have a backup in case the battery dies and I don't notice.