The Glock is a safe firearm. The way it's designed, it's idiot-proof (not attorney-proof, however), and simple to operate under stress. Add a manual safety, and that is something else that could go worng at a bad time.
I don't intend to add one of these safeties on my 22, as I am comfortable with it as is.
Jesse, thanks for posting this. Appreciate the link, too. I read about this last month in an advertisement in a gun magazine and wondered what others would think. I talked with a few of the instructors last night about it and they are in agreement that it may be a waste of time.
Dennis, just remember that anything mechanical can fail, no matter what the design is.
If you need a manual safety, buy another semi-auto with one. Otherwise, you're asking for a headache by modifying a Glock.