It seems like there are more and more books where part of the Earth is mysteriously changed overnight. S.M. Sirling's Islands in the Sea of Time and Wilson's own Mysterium come to mind. In all of them that I have read, you simply accept this change as part of the setting of the story. Darwinia is different, in that the main character wants to find out why the change occured, and much of the story is dedicated to explaining that change, and this was why I liked this book better than I expected.
The story was pretty slow at the beginning, and I almost gave up on the book. However, once Wilson started to explain how the change happened, it got much more interesting, if not much faster. I enjoyed the characters in the book, particularly Gulliford Law, who is curious about what happened to cause the change in the Earth, but once he finds out, wants nothing more to do with it. As I mentioned, the story is kind of slow, but I think it is worthwhile.
I have read several books by Wilson, and they have never been quite what I was expecting going in, but I have enjoyed them all. I would recommend this book, and will look to read more of Wilson in the future.