I use the function keys a lot for changing volume, screen/keyboard brightness, pausing/playing music. I also launch Terminal and Firefox to F3 and F4, I heavily make use of ESC for all kinds of things (VIM, closing UI dialogs) and I occasionally use the function keys for some shortcuts such as finding the next occurrence with F3 in Foxit Reader, F10 to compile, F7 to show the desktop, F6 to switch between Karabiner profiles. For me it would be quite a loss.
