Works as described. all your concerns are true. - LaserMax Lightning Rail Mounted Laser Sight w/GripSense

Gripsense technology looked promising since my normal go-to brand does not make the laserguard with instant-on for my current CC arms.  My concerns are the same as you might be thinking, so they will answer in this review.   

Does it really work?  Yes, it does. 

Does it stay on while in the holster? Yes, it does. 

Does it turn on while you walk? Yes. 

Will it go on if you place the gun down? It will go on if you place it sensor side down.

Does it work with a left-hand grip?  Sometimes.  It will work with a left hand grip on my XDM & XDs. It will not work on my H&K P30. 

Does it work on H&K P30? No. The paddle magazine release seems to block the sensor. 

Will it fit in a holster with the laser? Yes. A non-fitted leather hoster can still be used with no modification needed.  (The laser does not pass the trigger guard.) 

This green laser is the BRIGHTEST I have ever seen. (Compared to my last 2 CT in green.) 

I would recommend this as a nightstand gun option.  It will work just like the promo video. It will not work if you lay the gun sensor side down. You can shut the sensor off and use the buttons however, this defeats the purpose of getting the instant-on tech I (you) wanted. 

I might just have to get a Glock since there are 100s of accessories for them and very little for Heckler & Koch. If you don't need the instant-on, this is a great, super bright, and compact rail laser that fits on any compact to full-size pistol.

PS- This comes with 3 different settings for sensitivity. Your pistol may work on a lower setting. My pistols did not...