As I said in one of the comments to a previous post, this is an idea that Terry had in the eighties, wrote a large chunk, then found that his publishers wanted him to concentrate on more Discworld since sales were pretty much guaranteed. Baxter seems to have done a reasonably good job of writing around the bits that Terry did (and no, I don't know where things begin and end, except that I spotted a few ideas that Terry told me about back then, such as a church that worships God as a confidence trickster).
If you want something to compare it to, the Merchant Princes series by Charles Stross has a slightly similar premise, except that in The Long Earth most of the world's population is travelling into parallel worlds that are initially empty of humanity - one of the central ideas is that intelligence is evolutionarily very unlikely, and evolution is a lot more chaotic than we generally think, so timelines with intelligent life are extremely rare. There are intelligent alternatives, just nothing human.
The plot is basically a voyage of discovery, with some action, stories of how people adapted to the changes caused by dimensional travel, and occasional humour. It isn't going for continuous comedy, as in the Discworld books, but it was never meant to. It ends with the main problem of the story resolved, and a slightly clunky "come, Watson, the game's afoot" riff at the end to possibly pave the way for sequels.
Bottom line, I enjoyed it - it's not going to change anyone's life, but it's reasonably good fun. Recommended.