The Janus Conjunction, although it starts out well, is a terrible novel...The character of the Eighth Doctor is unconvincingly written and completely off - as a matter of fact, he's almost exactly like the Third Doctor, but much more annoying. Sam's character is also done terribly. She contributes nothing to the novel and only serves to annoy the reader. All the characters in the story are flat and boring. Even Lunder, the only close to 3D character, you begin to hate after awhile because of his attitude.
The plot works, but it's very pedestrian. It's just a bunch of running around back and forth between Menda and Janus Prime - nothing particullarly exciting happens. The radiation melting the soldiers is just a new twist on an old idea (from page 18 of Vampire Science). There are some serious scientific errors in the resolution of the novel concerning the moons and the weapon, but by the time you reach that point in the book, you will probably only care that the book is almost over.
Overall, avoid this book at all costs!!! Perhaps this would have made a good Third Doctor and Jo story, but it's absolutely terrible with the Eigth Doctor and Sam. It's not a pain to read through, but by the time you finish, you WILL be thoroughly dissatisfied.