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Apart from it being really fun to read, it's description of ever shifting deadlines, continuously changing specifications and numerous code re-writes is so recognizable to me as a developer. Just check out Joel Spolsky's review to see what I mean.
And what about the software itself ? Well, I haven't tried it yet but the concept is great, though not as revolutionary as it might has seemed in 2002. As already mentioned in the book, 37Signals already provides most of the functionality as a web application. With all the benefits: no server maintenance, available everywhere through every browser and automatic upgrades.
But the Outlook / Exchange combination is still used in many corporate environments. And since Chandler is aimed at replacing that, they might still have a chance.