Back before Terry started writing about giant space turtles, elephants and wizzards, he wrote science-fiction. Strata and Dark Side of the Sun were two of Terry's earliest novels, after which he left the genre (although some people would argue that later Discworld novels are bordering on the science-fictional, and certainly speculative fiction, at least within the confines of the Discworld itself).

The Long Earth was announced in 2010 as a collaborative project between Terry and Stephen Baxter - a prominent sci-fi author who I freely admit I had not previously read. The central theme was to be parallel worlds and the implications this brings to humanity. As it turns out, there's also discussion about the freedom of information, the effects of privilege, a mystery to be solved (it can't be a Pratchett without some kind of mystery) and a questionably human intelligence trapped in a computer. Which, to be fair, does distinctly start to sound like a Discworld novel.

Our protagonist is a loner - unusual in the circumstances of his birth, he has a disconnect from crowds of humans and spends most of his time in the higher parallel worlds. He is called for a meeting with the Black mega-corporation and meets Lobsang, a Tibetan motorcycle repair-man reincarnated as a computer program and agrees to accompany Lobsang on a journey of discovery through the higher outer worlds.

So, the good:
The science is novel, and good. I like the idea of the Stepper, and harkens both back to build-your-own computers such as the Z80 and such current technologies as the Raspberry Pi and Arduino. The Stepper itself may or may not be directly necessary for stepping, but the building of the object and the almost meditative quality of putting the components together has a distinct psychological quality.
On the science, I found the handling of parallel worlds excellent. The idea that the parallels are similar which small divergences rings true, as does the idea that higher in to the outer worlds the higher the divergence. I also like the idea of Joker's - which reflects on the idea that parallel worlds are probably chaotic - which is also perfectly sensible.
Lobsang is clearly a Terry invention. The idea that he tries to evolve physically (although I think the emotional evolution is flawed) works well, and although we never see behind his "blue door", I suspect there's more of him behind there than he lets on to Joshua.
I loved the idea of trolls and elves. And given stepping, communal memories and a few other traits, there's a whole wealth of ideas here to play with. It also fits nicely into existing mythology and the inability to step with iron makes some of the fairy stories fit in to this universe.

The bad:
I found the pace testy. The middle section of the story drifts, and for me, stopped being a compulsive page-turner. Admittedly, some of this pace returns towards the end of the book.
The conclusion to the mystery part of the story felt like a big deus-ex machina. I'll be honest, I'm not convinced it really works. However it does provide a significant turning point for one of the characters.
The characters don't really develop. By the end of the novel, there's no major character progression - they are all pretty similar to how they started. I realise that this book is intended to have a sequel, I may revise this opinion once the sequel is released.
The trolls and elves aren't really given as much action or interaction as I would like to see. Two alien, intelligent races are introduced, divergent from human evolution, and despite some pretty light interaction, they're not really worked with as plot elements. I would have loved to have seen more time spent with them, ideally building relationships, exploring their nature, how they interact with humans. Considering they're relatively central to the set-up of the mystery, they feel like very external characters to this story.

Overall - I've enjoyed it, despite the negatives. It's a new world, and I'm a little forgiving of one of my favourite authors. I can't write for Stephen Baxter's influence on this, I'll need to read some of his work before I can comment. I would like to see more exploration of this universe, and await the sequel.