It mimics the gmail look & feel, i.e. one line of text represents one thread in your inbox, and when you open it, you get all the messages of the thread (with read messages collapsed), instead of precisely one message.
Edit: also fast (!) full-text search thanks to Xapian, tags instead of folders
Yup. I do this since I'm used to za in Vim for toggling folds:
# Collapse all threads by default, except if
# there's new mail in one.
unset collapse_unread
folder-hook . "exec collapse-all"
# Toggle collapsed threads with za, like in vim.
bind index za collapse-thread