Worthwhile for extensive reading

There's a large stretch of this novel at the end which is rather brilliant, and in general I found the entire last half to be rather gripping. The first half, however, is quite slow, and the whole book is a challenging read. Not challenging like Shakespeare's Julius Caesar, but rather like an overly literal and scholarly translation of an ancient text where you have to look at the footnotes at the end of every sentence...
Part of the problem, if you ask me, is down to the ideas being very visual. The book is adapted from a TV story Marc Platt pitched to Andrew Cartmel that was rejected on budgetary grounds. Honestly, I could see a TV version of this being brilliant, but it just doesn't come across well in prose.

Marc Platt had previously written Ghost Light for televised Doctor Who, a very interesting story that I defy you to understand on your first viewing. It's very dense and clearly it's about a lot of things; what those things are, I'm still not quite sure to this day (but then again, I've not seen the extended version, or read the novelisation, or the shooting script). This novel doesn't suffer from the same time constraints of Ghost Light (quite the opposite—as I noted, the first half feels quite slow), but Platt still isn't interested in dwelling on any idea or explanation for longer than is strictly necessary, so if you do get confused, don't be afraid to look back over what you've read and take a moment to think about it before moving on.

This book is a worthwhile read, despite sometimes being a bit of a challenge. It is quite confusing in places, but at the same time, this is the first time we get any clear insight into Gallifrey's history, and my my, what an insight. We learn a lot about Rassilon and his rise to power, and we get hints at the mysteries surrounding the Doctor that came up during the Sylvester McCoy era of televised Doctor Who, which will continue to play into the VNAs from this point.

I think, really, the biggest flaw with this novel is that it directly followed Timewyrm Revelation. If it hadn't, it would be seen as a revolutionary experiment in what a prose Doctor Who story can be.

That said, I can't honestly imagine a version of the A-plot written as a novel that would have come off particularly great. The Gallifrey-focused B-plot is absolutely riveting every time it comes up, but I don't think that makes up even one sixth of the published book's length.

I can't recommend it as a standout, but its ideas are interesting, and the whole B-plot is glorious.

Recommended background: Remembrance of the Daleks (season 25)
Worthwhile background: An Unearthly Child (season 1; watch Part 1 only), the Edge of Destruction (season 1), the Three Doctors (season 10), the Brain of Morbius (season 13), the Deadly Assassin (season 14), Arc of Infinity (season 20), the Five Doctors (special), Silver Nemesis (season 25), Remembrance of the Daleks (novelisation)

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.