Although I have about 40 pages left to go, I'm really enjoying this book. The brief summary I read beforehand doesn't really give an accurate idea of what the book is about - as I've moved from chapter to chapter, it's easier to see that the author is explaining more about how the mind works than about happiness. I would say the basic premise of the book is that just as our minds fill in gaps in our past experience (memory) and in our visual experience (sight) with material from the surrounding present, our minds also use that material to build an idea of the future - specifically our future - and that's why so often we expect to feel one way in a given circumstance and feel quite another way when we have actually experienced it. There are dozens of cited studies that help explain the ideas the author builds upon, and I think it is overall a very well-researched book. Gilbert puts forth some curious theories, but all of them have a lot of support. While I don't plan on accepting everything he says as pure fact, the book has changed my way of thinking about thinking. At least for a while.
Rating: I'll probably never read the whole thing through again, but it would be nice to own a copy for reference (and for having a larger library, of course).