The Ortlieb mount goes on easy and stays on. It's a solid mount. And your Garmin mount will work with the Ortlieb mount.

The complaints I've seen have been from people who wanted to take the mount off and on. It can be done, but that can also booger up the cable and you'll need to buy another one. I leave the mounts on -- they don't weigh anything. (I also leave both water bottle cages on even for short rides that only take one bottle.)

I did get a secondary stem for my travel bike, which needs to be taken apart to fly. Again, the mount stays on the stem.