I have the 77mm, a M50 1.7 and a Takumar 55 1.8.
My main field is portrait photography.
With the 77, while it is a dream of a lens, i often find myself quite far away from the model, especially if i want to show not only the head but also shoulders or upper body.
When i'm that far away, i feel that the connection between me and the model is not as it should be. A 50 comes closer to the distance i would also have with a person that i'm not taking pictures of but i'm just talking to in a social situation.
Also sometimes, i get the 50 out when i'm bumping into the wall with my back when shooting indoors :-)
I would love to get the upcoming full frame camera, just so that i can use the 77 from as close as i can use a 50 on APSC.
Hope that helps
Bromberger