I agree it's a very nice piece of kit if you don't mind the price. Most of the time I run on the low reactive beam, which is enough for most trails. For very rooty/rounded granite trails I turn it up, mostly just because I can 
On transition to trail it goes dim under every streetlight, which I suppose is OK and as it should function, but it takes a bit to get used to. Also in colder weather an out-breath causes a temporary rise in reflected light, which is sensed and the light cuts down quickly. This can be a bit off-putting but not risky. On can set to constant output at the two brightnesses if this or other conditions like fog or falling snow make this an irritation.
The stringy headstrap work well, much better than I expected. I find the weight acceptable (compared with my former 3 x AA) as it is a single custom battery at I assume standard 5V for USB charging. The nattery level indictor is very useful to have.
I find the twist-and-springback power button the easiest to use of many headtorches I have tried. It would be easy to use even with thick lobster gloves on. The twist cycles off-high-low-off and a long hold ( 2 secs) flips reactive constant. Just a slight niggle is to have to go round the cycle to get from low back to high.
I like the fact that a waist strap is even available, and in principle this gives better shadow relief for roots, etc. but I manage OK so far. My SO is very jealous of it 