The idea behind the story was interesting, but I didn't really like its execution. I felt the same about the TV episode when I watched it. It was a good story, though just not my type. I didn't buy the world of Marinus either.
The best thing about this book: female characters finally saving the day. Both Barbara and Susan took charge of things and did something more than just staying there and be saved. This was, actually, the first episode in the TV series I started to like Barbara. And I still don't like Susan (I watched all her episodes until she left), but she did something useful for a change during the Vasor sub-arc, and I liked it. I wish the writers had given her more chances to show a smarter and braver side rather than being just a silly child. And then we got Sabetha, to whom the narrator always referred as having the same aura of leadership as her father (I don't remember the exact words, but that was the general idea). Yes, for a reader in 2013 it doesn't add anything new, but if you consider the context and compare it with the previous stories (and maybe the whole season 1) of Doctor Who, you'll find some progress here. For a 1960's story from mainstream media, I give it credit. Yes, Ian still keeps saving the day after all and being the great white-male-hetero hero (I like Ian, I just didn't like how gender-based they wrote his and Barbara's roles in most of the storylines), but at least we had Barbara taking that role at least once.