If you're a fan of ENT, you will really appreciate "In a Mirror, Darkly" more after watching the TOS episodes "Mirror, Mirror" and "The Tholian Web."
When you're done with TOS and find yourself wanting more, don't hesitate to check out The Animated Series (free to watch on StarTrek.com and also on Netflix in HD). It featured some of the same writers and most of the original cast did the voice work, and had all the freedom of animation to depict aliens and alien worlds in that classic 70s style. A lot of people consider it the highlights of years 4 and 5 of the famous 5 year mission of the Enterprise.
And if you REALLY like TOS, you can also check out Phase II (aka New Voyages) on YouTube, which picks up right where the last TOS episode "Turnabout Intruder" leaves off and even has some appearances by TOS cast members.