I've got mixed feelings about this episode. On the one hand, it was enjoyable seeing areas of the new TARDIS, which included a glimpse at the swimming pool, the library, an observatory, and even the Doctor's store room of cherished mementos. On the other hand, the story was mediocre, which is to be expected from a show run by Stephen Moffat, and the ending was another "it never happened" cheat that abused time travel as a storytelling aspect.
Yes, I just gave you a major spoiler. Considering how awful it was, you're not missing anything by knowing how the episode ends.
Okay, the plot revolves around the Doctor, Clara, and a trio of space salvage brothers as they try to save themselves from hideous burnt-mummy-clay-things. At the same time the Doctor sets the TARDIS controls to a level Clara is able to handle, in an effort to bring her and the ancient time ship closer together, the salvagers lock a powerful tractor beam on the exterior, which happens to be materialized in space, heavily damaging the TARDIS. Upon waking up from under the junk pile where the salvage team's spaceship dropped the exterior, the Doctor then proceeds to threaten them to help him locate Clara, who is lost somewhere inside the TARDIS, by activating a self destruct sequence. And as is quickly evident, the five are not alone, and soon five become four.
There were a few nice touches, such as when one salvager removes a console panel only to hear the echoes of voices from the past, bringing us another reminder of the classic series, and another when Clara discovers the library and, irritated and impressed at the same time, reacts with the line, "now you're just showing off." Also, while there, Clara makes an interesting if predictable discovery that, really, shouldn't have been included -- at least not the way it was in this episode.
So we have yet another contrived ending that once again leaves us feeling robbed of what should have been an epic moment in the development of Clara's and the Doctor's relationship. I give "Journey to the Center of the TARDIS" a C- for poor writing. I give Stephen Moffat a big fat F for his atrocious handling of Series 5-7.