I put the RBIS three-spot mount on in January, thinking to mount phone, GPS (in AquaBox), and sometimes the GoPro. With all three spots in use it's pretty crowded if all you have are the standard-length mounting arms. You may want at least one long arm, to use in the center spot.
Depending on what you put where, and how, you may find minor clearance issues with the windshield in the full down position, and partial line-of-sight obstructions to the instrument panel. I'm still experimenting to figure things out, but the GPS has kinda removed itself from contention, for the time being at least, for technical reasons unrelated to this topic and the GoPro doesn't go on most of my rides. The phone mount (the spring-loaded "X" type) has become a permanent addition, using the right hand position.
From a mechanical standpoint, the mount is solid and stable. I do find that it's a little easier than I'd like to loosen the mounting balls; next time I'm under the dash for something I'll tighten them up firmly after adding a couple drops of blue LocTite to the threads. Mounting the thing was simple and straightforward.
Overall, I'm quite happy with it.