After reading the final page and closing the book, I felt like I read 800 pages of a story that I didn't care too much about.
OK, Brin does a good exercise in imagining what would be the Planet's conditions fifty years after he wrote the book. But that's it. In the middle, just so the story would have a plot, there's the thing about the black hole graviting through Earth's central core, and the struggle of a team of scientists to stop it and to find who was responsible for it.
I think Brin tried to hold all the world with this book, and came up with a flat story. Yes, it's nice to read some of the passages concerning ecological development in the Planet for the next half century, but that was about it. The characters appearances were so spaced and so inocuous that I didn't feel related to them; in fact, in the end I couldn't care less if they were able to save the planet or not. There's something else with Brin's books: he uses some complex concepts and explains very little of them. Sometimes the reader, being ignorant about the science facts that he's reading about, stops trying to understand, and then the story looses most of its fun. We could see this problems, altough in a minor scale, in books as "Startide Rising" and "The postman". I think Brin likes to create his stories trying to pass to the readers a sense of mistery that sometimes doesn't work well in science fiction.
"Earth" could be better developed in lots of aspects.