I ordered a BT21 from Gearbest around the 21st and got it within a week.
Took it out for a first ride last night. I’m running it paired up with four good quality protected XTAR cells in a battery box. It survived getting rained on, and is plenty bright enough. I only used the overdrive mode on fast descents and suspect it won’t do much intelligent stuff to mitigate the heat buildup over time. It does get hot fast, so we’ll see if the driver burns out in the next few months (a similarly built one sold as the Jexree Owl seemed fine at first, but has got noticeably dimmer over the year to the point it’s useless now – whereas I’ve had years of use out of P60 based XM-L2 and P7 flashlights).
One of the things I liked the look of with the BT21 was the heatsink giving a decent mounting area at the side of the light. I hate the rubber band light mounts these all come with, so bought a Gloworm quick release mount off the classifieds here, and adapted the light with a bolt, a range of drills from 2.5 to 5mm, and an M3 tap:
The messy looking spacer is there for now because I only had long M3 machine screws to hand. The first fin has a 5mm hole to accommodate the inner facing part of the mount, the thread goes all the way through the rest.
Gloworm mounts are the best I’ve seen: machined aluminium, low footprint, can raise the light up a little to help fire it over cables, and let you position the light body directly over the stem (which is more an OCD thing for me than anything, but does take up less room on the bars).