I've tried to like it and believe that instant feedback is actually useful but no. I couldn't.

Emacs IEdit mode has done this for years

How is immediate feedback and interactivity going to help if the changes span more than one page? If you are going to extrapolate(looks fine on this page, must be fine elsewhere), you can very well extrapolate from a smaller sample size(run :s/some_crap/some_other_crap/gc, check one or two occurrences, then do 'a' for all).