Well, it takes a lot of patience with the limited budget and techniques they had to work with at the time. I'm quite fond of it, but I grew up with it. The animation is limited and the performances from Shatner, Nimoy, and Kelley tend to be pretty flat. But it is as faithful a continuation of TOS as they could make it, with the whole main cast returning except Walter Koenig (who did write an episode), and with half of its episodes written by veterans of the original show. D.C. Fontana was the story editor for most of it, and she strove to tell stories on an equally sophisticated level to TOS, albeit with less sex and violence and with much more freedom to show exotic aliens and settings and tell stories too expensive to shoot in live action. If you can look past the production limitations and focus on the stories, you may enjoy it. It has a lot of mediocre episodes, but there are some classics there, notably D.C. Fontana's "Yesteryear."
If nothing else, it's a part of Star Trek history that deserves to be acknowledged, the one followup to TOS that had the most creators, actors, plots, and settings in common with the original show. It brings back Sarek, Harry Mudd, and Cyrano Jones (with the same actors) as well as Amanda, Kor, Koloth, and Bob Wesley (without the actors). It's the debut of Robert April as the first captain of the Enterprise (rather than just a rejected name for Pike) and has the only onscreen mention of McCoy's daughter.
