The Arca-style tripod heads and plates are one of my favorites. I use them now for over 4 years, and I guess I'll be using them for a long time. Especially the so-called L-plates are awesome.
The L-plates are plates which enables you to put the camera in portrait orientation on the tripod head, without putting the top of the head in an awkward vertical position, which lowers the effective height of your camera on the tripod.
For my former Nikon D300 I had a L-plate by Really Right Stuff, and now that I upgraded to a Fujifilm X-T1, I needed one for that model. This time they (Really Right Stuff - RSS) created a modular L-plate. The former D300 version was made out of one piece, but the this one allows you to remove the L-part of the plate, making the camera lighter. So you need to add that part if you intend to shoot in the portrait orientation. The good thing is that you can order the parts separately. So you can start with the base plate and get the L-part when you need it. I just got them both at the same time.
The entire kit comes with the appropriate hex wrenches and a small screw which can be used on the bottom plate as a stop, so the camera won't accidentally slide out of the ball head. Unfortunately, there's only one stop screw available on the bottom, so the camera can still slide to the other side.
The connection of the two parts is rock solid. No movement what so ever. I just hope that it doesn't wear over time.
While the L-plate is attached to the camera, you can still access all the important parts of the camera.
There is one downside to the L-plate. You cannot use the Fuji wired shutter remote when you have the l-part attached to the bottom plate. But you can always use the Fuji smartphone app to remotely control your camera via a wireless connection.