Update:
Thanks to surebet for his mention of the new G&P M500
... I got very interested again. So for $89 I got the New G&P M500. Sorry, their not the best pictures...
You actually can have a constant on... At least on mine. Both the pressure and the rocker switches work perfectly fine and the way it's suppose to. the rear rocker click turns it on for constant and forward switch to turn off.
The unit is definitely noticibly heavy but it's not too bad and the only cons I can think of is that it does make the M4 front heavy and the bezel does get quite hot just after 30 seconds of turning it on. I haven't really used the light yet so I don't know the battery life. I haven't used this long enough to recommend it yet.