How long did it take you to convert? Did you relinquish vim key-bindings and cross to straight emacs? I still prefer out of habit vim keystrokes, but I'm having to relearn everything else (buffer navigation, searching, definition navigation etc)

No benefit of relinquishing vim keybindings, but you have to learn some extra emacs bindings (like Ctrl+G to cancel) if you need to handle every situation.

Which both worlds?

The extensibility of Emacs, along with Vi[m] key bindings.

Although Spacemacs does have a "Holy mode" which will keep the Emacs bindings and won't use vim-like keys.

Probably modal editing and plugins that aren't constrained by Vimscript.

Presumably Emacs and Vim.