This is a good episode for its further exploration of Spock's background, highlighting the conflict between his human and Vulcan parentage. It fits nicely into the established canon from Journey to Babel, by showing the pet sehlat that Spock's mother Amanda mentions in that episode. It adds significantly to our knowledge of Vulcan, both the planet and the society. And Mark Lenard even returns to do the voice acting of the cartoon Sarek.

On the other hand... the Federation's use of the Guardian of Forever is silly, if not downright reckless. After the events of The City on the Edge of Forever, do we really believe that:

1. Kirk would not only willingly go back to the Guardian of Forever, but would undertake a time trip with such evident enjoyment, after the first trip broke his heart with the forced death of what may well have been the one true love in his life, Edith Keeler?
2. The Federation would allow, much less sponsor, historians going back in time to observe past events, when we know from what happened to Kirk that some small random change could result in Hitler winning World War II?* Or possibly something even worse? Especially given that the innocent observation trip that Kirk and Spock take at the beginning of this episode - to a completely unrelated planet, mind you - manages to alter time sufficiently to erase Spock from existence on Vulcan? Wouldn't you think the Federation would instead have a planetary blockade armed to the teeth to prevent anyone from ever going near that damned Guardian??

Don't get me wrong, though. Those failings aside, this is still a good episode.

* Remember the First Law of Time Travel, as quoted by Spock: "Time travel: ergo, Hitler."

Transcript

1
Captain's log, Stardate 5373.4. The Federation is using the Guardian of Forever for historical research. I've assured them it's perfectly safe.
{Kirk and Spock stepping out of the Guardian}
Kirk: What a trip, Bones!
McCoy: Who's this Vulcan with you, Jim?
Spock: I have been erased from this timeline!
2
{Enterprise briefing room}
Uhura: Records show Spock, son of Sarek, died at age 7 in the kahs-wan maturity trials.
Spock: No, a mysterious cousin saved my life.
Kirk: Did he look like you?
Spock: Why... yes.
Kirk: Allons-y!
3
Caption: The past, Vulcan:
Spock: Myself as a boy.
Vulcan Boy 1: Emotional Earther!
Young Spock: Why you...!
Vulcan Boy 1: Fighting isn't logical, but teasing you is, Spock!
Vulcan Boy 2: Ha ha!
Caption: Cool Vulcan shorts.
4
Caption: A le-matya attacks!
{Spock jumps on its back and grapples it}
Caption: Neck pinch!
5
Young Spock: You saved me, cousin Selek! Can you teach me the Vulcan neck pinch?
Spock: I see no reason why compounding a temporal paradox will cause any further problem.
6
Caption: Personal log. I have saved my younger self and restored history. Fortunately the risks of time travel can be rectified.
{Spock at the Guardian of Forever. McCoy is in a Nazi uniform, saluting}
McCoy: Spock! Welcome back. Heil Hitler!