The first chapter is gorgeous. I have no doubts about saying that it's one of the best chapters I have ever read, and is certainly enough to make a grown man tear up a little. It really does remind me a bit of East of Eden. The story of the American Chestnut is unbelievably tragic, and Powers does an amazing job creating a sense of longing for a landscape you will never see. The next few chapters introduce the other characters, and are all at least interesting enough. The wheels start to fall off about a third of the way through, and there is a single event that totally kills the momentum of the book in my opinion (you will know what I'm talking about if you read it). The last part often devolves into melodramatic plot points, but there is still some good with the characters that remain separate from the main group.
The Overstory is certainly worth reading for the first part alone. The prose remains beautiful throughout, and if you're actually interested in the biology of trees then it's not bad in that sense either.
The first chapter is so good I was willing to give the whole rest of the goddamn book a chance before realizing there's not much to it but misanthropic navel gazing.
I think it could have been better as a short story collection, maybe? There really isn't much to most of the characters' stories after their introductions, the only ones I feel like had even a bit of interesting development later on were the professor, the disabled game developer, and the lawyer couple.