Essential reading.

With this book, I wondered whether I should revise my ratings system. It's transcendentally good; even if you have no plans to read the VNAs as a whole, even if you had no interest in reading any of them, this one is a must read if you like Doctor Who. Even casual fans would do well to read this book, it's that good.

There's a small reference to the very first Doctor Who story, which I have excluded from the extensive background for the reason that it's a reference to one line of dialogue. If you're a new series fan, you may still get the reference, since the line was referenced once again in the Moffat era. I think this, and a few moments and ideas across the Davies and Moffat eras of the new series, were in fact due to both showrunners really liking this book.

The first three books of the VNAs were an attempt to do some more Doctor Who, in prose form. This one explored just what Doctor Who could be, if it was free from constraints of budget and time, and did a story that was aimed squarely at the adults in the audience—not by deploying gratuitous sex or violence, but by being a legitimately mature and nuanced piece of science fiction. And it is absolutely goddamn mental.

Essential background: Timewyrm: Exodus (VNA \#2)
Extensive background: The Daleks' Master Plan (season 3 - 9/12 parts missing), Inferno (season 7), Earthshock (season 19), Remembrance of the Daleks (season 25), the Greatest Show in the Galaxy (season 25), Ghost Light (season 26), Survival (season 26)

In my reviews, particularly of the Virgin New Adventures, I prefer to use a 4-tier system of grading:
Essential reading - If you want to read all the best VNAs, get all the most memorable story arc beats, and generally enjoy the VNAs without having to trudge through the mediocre/bad books, or perhaps even if you just want to pick up a good Doctor Who book with no intention of reading the entire series, look for this rating.
Worthwhile for extensive reading - Not outstanding, but I won't outright tell you to skip it if you want a sense of the VNAs overall. If you're determind to only read the best, skip these, but for a read-through of the series, I wouldn't skip them. They're the worthwhile, good-but-not-amazing books. You'll get a stronger sense of character arcs, story arc beats, and the growth of the VNAs as a range if you read these, but it will also take you a lot longer.
Not recommended - Not very good. If you really want to maximise your experience of the VNAs, you could read this, but it's definitely not advised.
Avoid at all costs - An irredeemable lump of human fecal matter. Do not waste your time with this insult to the franchise.

In addition, I list Recommended background that you may find necessary for understanding the story in full, as well as Extensive background for some additional details you may find interesting.