The Doomsday Machine was a great episode when I first saw it as a tyke. It was an amazing episode when I watched it as a pimply teen. It was a stupendous episode when I watched it as a young adult. And it remains my all time favorite episode as I watch it now at 45 years old. The original effects take nothing away from the show, they never did. Bad or old effects don't impact negatively on my viewing experience because a) they do the job and b) I watch every TV show and movie well aware of the limitations of the time and fully appreciate they did the very best they could with the time and money they had. I never needed new effects in Star Trek. Ever. It's great as is. I'm also more forgiving of "bad" effect made 50 years ago than I am over bad effects made in 2006. At least 50 years ago they had an excuse.
Hell, I don't even watch the HD prints most of the time. It's lower res laserdisc for me.