Considering the origin of this story - that they had to hastily put together a 2 episode serial to give the BBC a stopping point if they did cancel the show after 13 episodes rather than stopping in the middle of Marco Polo; and also had to be done with basically no budget - this is a remarkable good story. Its also interesting for there being no real villain - indeed the main problem is simply the Fast Return switch being broken and causing the TARDIS to malfunction, setting up jeopardy in many future expanded universe Doctor Who stories.

The main highlight of this story in my eyes is in developing the relationships of the characters: this feels like the ultimate end of the somewhat-antagonistic relationship between the Doctor and Ian+Barbara. While the obvious impacts of the TARDIS console malfunctioning is on Susan (who spends the first episode threatening Ian and Barbara with scissors; being clearly sceptical of their motives) and then the cliffhanger setup with Ian it impacts the Doctor longer than anyone else; and its only the fault locator providing additional information that helps the crew unite, stick together the various chains of information they have to learn the issue at hand.

I guess downsides might be around the pacing - although it moves quicker than the Daleks - and the fact that a TARDIS-bound two partner with no villain is unlikely to be quite as exciting; and the natural conclusion (the fault being a broken spring in a switch) is somewhat anti-climactic. Overall though I really enjoyed this story; and as an ending the 13 episode initial run until we get to the first lost episode it works well.