It's a very different experience.
Some of us who use Vimperator use it not only because of the key binding, but also for the command-line, ability to modify the UI (e.g. :set gui=none), ability to put .vimperatorrc in .dotfiles and use it across multiple systems, and such. As far as I know, VimFX still not capable of all these functionalities (some due to difference in mission, e.g. not modifying the UI.)
That said, WebExtensions at its current state still couldn't support all of VimFX's current feature either.[1]