I get very few channels and had not seen any of the original series in quite a few years. I had liked it when it first came out.
I have been a little surprised by how many weak episodes there are but there really are some very good ones too. I think the main thing which made it good was simply the actors, even Sulu. They also seemed to be genuinely good friends.
I have been watching the original series on the Heros and Icons channel, and never realized just how many bad/preposterous episodes there were. Spock’s Brain is high on the list, but there are many others that were just downright awful, like they had a deadline, but all their writers were out sick with the flu.