I carry one with me and use this sometimes with my DF and it works well enough when you need it such as when shooting low to the ground etc. However, the shooting experience is definitely different to that of using a big bright WLF such as the one in the RZ which projects an almost 3D rendering of the scene that snaps into focus. With the FA401 you are very aware of the fact that you are using a turret type of viewfinder and I find that I have to move my eye to see into the edges of the frame vs seeing everything in a single view.
The finder is pretty easy to mount in the field with the only tricky part being removing the rubber std viewfinder if you have one of the newer ones with the lock on it. The FA401 latches on to the body with a clip and stays on pretty firmly plus rotates to whichever angle you need. The 'rubber' eyepiece is cheap and nasty but functional plus you can adjust it for your eyesight.
So, does it work and do the job? Yes.
Would I compare it to using a full size WLF experience? No, not really.