It’s been awhile since I broke out the TOS and watched it on Blu-ray but I seem to remember the original effects scenes were of pretty decent quality and blended in well with the surrounding material. I was very impressed with how they handled the whole “upgrade” situation. To be honest, I just watch the upgraded versions usually because they upgraded it with tender, loving care.
As a Star Wars fan, I dearly appreciate the fact that they were as interested, just as much as I am, in keeping the original footage available though.
There was an interview with “Rod” Roddenberry, I think on CNN or CBS when the remastered sets were about to be released and he assured the newscaster and the viewers that unlike the situation with another well known Sci-Fi franchise, the CGI upgraded versions will always exist alongside the unaltered originals.