I'm not really a fan of this one, personally. I don't think it was awful - there were bits I liked - but overall I didn't find the plot very exciting or entertaining.
I enjoyed seeing the Doctor's character develop further, in that we see a more affectionate side to him, and it's the first time we see the Doctor show romantic feelings (odd that people complain about that sort of thing in New-Who but it's present even in the very first season
). Great performance from Hartnell with those scenes.
I also enjoyed the focus on Barbra and the development of her character, too. Certainly the most entertaining part of this story was the fact that this is the first time they try to rewrite time.
Other than that, I didn't really enjoy this story. I'm not particularly sure why, I just think the overall feel was underwhelming and not as fun as I anticipated it to be, especially after hearing only good reviews. Maybe it's just me, but I'd give this one two out of five stars. I don't think was an awful story, like I said, but for the most part I didn't get much of a thrill from this one.