If you ever wish to take a photograph of a child, while (s)he is awake, the AE function is very helpful. I find the R3a finder on the 50mm setting to be wonderful.
The 90/40 setting is great too, but I use the 90 more than the 40 on the R3a, I prefer the 40 on a leica CL, the R3a 40 lines are a bit out there even without glasses.
The 75 setting is great too, big and clean.
Someday I wish I could figure a way to convert the shutter release lock collar to a meter on/off switch, instead of the partial depression of the shutter release to activate the meter. As it is now, in bright light the meter display is tough to see unless my eye is in the right place, so I sometimes try in vain to make it brighter by pressing the shutter release harder, thus taking various out of focus photos of the side of garbage cans and bushes etc... properly exposed though!