With 8 hours of constant use I would be concerned about overheating, possibly damaging the light and/or the batteries. Does seem like a good deal if you need a ridiculously bright headlamp, but I am wary of cheap lights.
I'm planning to get
this light by streamlight which is much less bright, but has a respectable 56 lumens and the fantastic streamlight quality I expect. It has regulation, but not very good regulation unfortunately(the output curve is far from flat). Nevertheless, if it's anything like my Streamlight propolymer luxeon it should beat any other headlamp I've seen.