I quick and captivating read, as it's actually a young adult book. The story is very Gaimanesque, the writing style not so much - it's obvious that Gaiman's partner in crime did most of the writing. I found the writing style a bit naive, it reads like someone's first book ever. Having read the bit about the authors in the back cover, it's obvious why; Michael Reaves is actually a television writer who wrote Emmy winning Star Trek episodes. Interworld was originally supposed to be a TV show, and it does seem a bit like a pilot - the big plot is solved in the end, but there are some lose ends left.
I like the concept of the book, it made me think that Buffy the Vampire Slayer would be a HEX show and Doctor Who would be a binary show, sort of...