While I can apprieciate that the M6 can sustain the M4's awesome light output for much longer, and although I really want one, I'm waiting for a rechargeable of equal power - the SureFrie M2000.
If SureFire make the M8, I'd buy that one, but I've only seen passing mention of the M8 on their site...
My advise, wait for the M2000.
Alastair