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I've not been out night riding as yet(hopefully trying that tomorrow night as it's to be dry-ish). But been using the mj872 for commuting, and I find myself deliberatly heading for darkness, tis amazing is all i can say. Only thing i have to compare it to is a shadey cateye thing i have.
Not had any overheating, stepping down issues at all. Though that might be different in 30 degree temps or something, i don't think it'll be relevant in scotland somehow!
For tomorrow night, I reckon I'll have it on the lowest setting most of the time and flick it up to 50% for the faster bits. 75% and 100% setting will only come in to use when it's raining or foggy I think.
As for rain, 2-3 weeks commuting and it's rained a fair bit, no issues what so ever so far. 2-3 weeks isn't a long test time, but i'm fairly confident the light will hold up well, seems pretty solid.
I found the MJ-872 overheated even with a decent airflow, and continually stepped down a level (sometimes two) until it cooled down
I'd imagine that major overheating issues (when most people aren't reporting them) is due to faulty build - some part of the heatsinking isn't working properly.