The first few minutes of this episode, with Kirk arriving on board a deserted Enterprise, looked to make for a very interesting and mysterious episode. Sadly, things soon really plummeted – the story was weak and awkward, with some real nitpicks and plot holes, and lacked the intrigue that should have gone with such a story.
This episode in many ways sums up why the third season is widely regarded as the weakest of the Original Series; In the first or second season, this would probably have made for an interesting if slightly filler episode, but here it is handled with little flair, and is not very engaging for the viewer as a result.
[Minor spoiler] The Enterprise, which turns out to be a duplicate, could be an interesting concept, but – as other reviewers have also picked up on – how could the people of Gideon create such an exact duplicate? It just wasn't believable. [End of minor spoiler]
Likewise, there is little real spark between Kirk and Odona, and by now the whole "Kirk and guest female of the week get romantically involved" device was getting very overused and worn out.
This episode holds little rewatchable value. While other episodes (such as the infamous "Spock's Brain", for example) may be even worse to watch, "The Mark of Gideon" certainly ranks amongst the Original Series' weakest episodes.