Even with a keyboard shortcut, I never found it to be truly satisfying. I cannot quite recall what was annoying, though. Maybe you cannot space back into the formula after closing it? (I certainly recall having the experience of typing a formula, closing it, noticing a mistake, hitting backspace, and deleting the entire formula.) Maybe there was just a slight delay for the formula editor to open? Also, $ is still more pleasant to type than Alt+=.

I would be willing to try again, but I'm not buying Word for the privilege.