duck - You asked for feedback, I gave you feedback based on my experience. My opinions and how I carry my Glock and use a triggerblock (or mostly don't) are formed from my EXPERIENCE with the thing, whether you "care" or not. BTW, the fanny pack I use a trigger block in is one I only use to transport the gun around in, not carry.
More:
How do they work? Very simply: they snap in, they snap out as advertised. Use the set screw to adjust fit if necessary, but do not over-snug the fit. You may need to sand it down some to optimally fit your individual trigger - mine sticks in one of my G19's. Do not manipulate the slide with one installed.
All-in-all, it will add a measureable element of awkwardness (and time) to your shooting because you are doing 2 trigger finger manipulations instead of one. It already takes time to develop a good draw from a holster, and be warned that these complicate it further.
In all candor, if you are only comfortable carrying a Glock with one of these installed, I would not get a Glock. It makes no sense to spend good money buying a gun that you honestly believe is unsafe in some respect as it comes from the factory. You will be more comfortable with a true double action pistol with a thumb safety, and probably a decocker. That means a Beretta or HK or Ruger or ..., I'm sure I've missed something. A thumb safety will be much quicker to release and does not pre-occupy the trigger finger. Proper trigger control causes perhaps more marksmanship problems than anything else, and that finger does not need any additional tasks.
If you do buy a Glock and install a trigger block, practice drawing and/or shooting ONLY with it installed, EVERY SINGLE TIME. Because if you don't, the first time you really need your gun you will forget to push out the block and then you're majorly, perhaps lethally, screwed. I've done it at the range.