I have two of these units, one in a G17 and one in a G19. I've had both for several years without the first problem. Mine must have been made in the middle of the week (as opposed to CavVet's Monday-or-Friday model [:D]) because mine are spot-on the smaller plate of my 2-plate spinner target at 10 yards (30 feet) - makes that thing sing and dance all day long...
edited to add:
Like CavVet said, when installing it in a Glock you replace the takedown bar with the new, included extended switch/takedown bar - left or right is on, center is off. This isn't a problem in most holsters, but can be problemmatic in a coat pocket, etc.
You don't mention the application in your original post, so please note that the on/off swith is different depending upon the firearm. For example: IIRC, in the Beretta, you replace the entire takedown-lever assembly - the new replacement has a push-button for on/off.
edited yet again for one last addition:
IN MY PERSONAL OPINION, if you want a laser for a Glock or similar polymer-framed pistol, the LaserMax is a good choice (internal, fits std. holster). But for a Beretta, 1911, etc. I would recommend the Crimson Trace Lasergrips instead (easier installation, less expensive, also fits std. holster, & ADJUSTABLE [although mine needed no adjustment out of the box]). I have CTLGs on two of my Berettas and one of my 1911s - I haven't had them as long as my LaserMaxes, but still no problems yet...