It's odd to read a TV Series Ace after getting used to the one from that later books.
Cat's Cradle -- as a series -- is not very successful. The books don't share much of any theme, and it's clear why the direct linkage in the titles was abandoned after this one. None of the books astounded me [even if Time's Cruicible is important to the Doctor Who mythos established in the New Adventures]
Not an astonishing book, but not a terrible one either.
Gareth Roberts is one of the highlights of the 90s Doctor Who authors. Like his Missing Adventures, there is nothing dark about his Doctor Who -- it's a Doctor who wants to use the secret weapon of "Diplomacy", a story filled with aliens with silly names.
I do have a few gripes about the book -- like Timewyrm:Genysis, Christopher has the Doctor "call up" the ghost of the Third. It may be easier to put Pertwee's Doctor onto paper than Colin Baker's one, especially if you want to use the stereotypical Doctor, but I think it's a cop out. It doesn't bother me if the Doctor uses skills primarily established with an another Doctor -- after all, the Sixth Doctor is the most physical after the third, and we really don't know the Sixth Doctor as well as we could some of the others.
Out of the three, I wouldn't necessarily say any are required Doctor Who readings; none of them are terrible, but none of them really grabbed me as excellent books either.