A pretty delightful experience overall.  I really enjoyed all the major elements of this audio, from the exciting start showing off the degeneration to the ultimate focus on the schemes of the Monk going up against the Fourth Doctor, Sarah Jane, and UNIT.  I get how some people might object to the "fan service" nature of this audio, featuring a number of cool cameos and performances throughout, but I, as a fan enjoying something built mostly just for fans anyways, enjoyed this story nonetheless.  The Black Archive and its history with the curator was a neat little background detail.  Where the story is weakest is actually around the villains, who are kind of putting the Monk up to his scheme.  These aliens are pretty boring and have a pretty generic voice.  I don't really love them or their angle of being monsters from an until-now unmentioned story.  It just didn't really work for me.

Everything else did though.  This version of the Doctor meeting Kate, Oswald meeting Sarah Jane, yeah, sure, why not?  I had a good time, I was entertained, I was interested, and that's all that really matters.