The other day I finished the first season of TOS. And you know what, I really enjoyed it. Some of the episodes still stand strong thanks to great writing such as The Corbomite Maneuver, Balance of Terror, and (of course) The City on the Edge of Forever. The effects do show their age though, especially with Devil in the Dark where the creature looked more like a giant melted pizza.
It is kinda interesting though that a lot of discussion of Star Trek today tends to overlook TOS in favor of TNG and DS9, at least when it comes to the fandom. I'm a little curious as to why that is.