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.