How could something that (in concept) seemed so perfect fail so miserably in its execution?
On the one hand you've got Michael Moorcock, one of the greatest living British fantasists, and undisputed master of the multiverse. On the other hand you've got Doctor Who, one of the greatest British fantasies, and undisputed master of the timestreams. Really, when you think about it, the only surprising thing about this crossover is that it's taken 48 years (both debuted in 1963) for the Eternal Champion and the Time Lord to meet.
They should have saved themselves the effort.
Coming of the Terraphiles is a nearly incomprehensible and entirely nonsensical bit of narrative fluff that does nothing to advance either storyline. Much of it is a Victorian farce surrounding the pursuit of a hat so enormous that wearing it would require the assistance of an anti-gravity device. What may have worked as a short-story length novelty is stretched, twisted, and drawn into into a novel that exceeds the novelty factor about 20 pages in.
The prologue is the best part of the book, packing more drama, suspense, and excitement into the first 6 pages than can be found throughout the remaining 330 pages combined. Even though Captain Cornelius is one of my least favourite incarnations of the Eternal Champion (give me Elric, Hawkmoon, Corum, or Erekosë any day), his story is infinitely more exciting than that of the Doctor. As much as I'd like to blame that on Matt Smith being my least favourite Doctor, and Amy being my least favourite companion, the fact that both are largely relegated to supporting characters (disappearing for multiple chapters at a time) makes that a stretch even for such a nonsense book.
Not nearly as witty as it wishes to be, or as clever as it pretends to be, this is the kind of fanboy crossover dream that makes you rethink being a fanboy of either franchise.