If it's one of their `900 lumen' (really more like 400 lumen) ~ $40 lights, mine failed too after a really bumpy section of concrete. Turns out the LED module had vibrated loose -- it's screwed in. I screwed it back in, and it's jammed in there pretty good now, and so far so good. It also hasn't had any problems with switching modes when I hit bumps since then. (I also put a piece of paper in it around the battery as a `shim' so it can't vibrate in there at all.

As for DE, I had a R/C helicopter blade pack with one blade breaking during shipping, and they gave me half credit for it -- which was acceptable, as it was half broken, but usable like that (get another pack, and now I have a spare.) Getting that required a little negotiating, however.

Either way, whatever light I use at night, I never ride at night without a spare light ready to go. The DE lights do seem less reliable than good quality dedicated bike lights, but even the good ones fail from time to time, and I really dread riding at night without lights.