I not only saw "Spock's Brain" first run (I was about to have my 12th birthday), it was also the first Trek episode I ever saw.
Have any of you read the Joel Engel biography of Roddenberry? The first episode he ever saw was "Spock's Brain" and he loved it. Something about watching McCoy slowly lose his brilliance as the effect of the Teacher wore off really spoke to him. Only later did he learn that "Brain" was generally regarded as the weakest episode. I think this goes to show that we all respond to different elements in a script. While there are some episodes that I consider sub-par, I can still enjoy watching them.
Still, Quasar is dead right. There are some episodes you just wouldn't share with your non-Trek friends. And he's right about not placing too much blame on Justman, who did much for the series. I just wanted to share what he had related in "Inside Star Trek".
I heard actress Marj Dusay (who played the Imorg Kara) thought Spock's Brain was considered one of the best episodes until informed otherwise when she appeared at a convention.

I would have loved to have been a "fly on the wall" at that appearance!
Heh, I'd like to meet her. I'd ask her how it feels to be a part of one of the (if not THE) most reviled Trek episode ever. I personally don't revile it...but as we all know, plenty of Trekkies do.