It's very well regarded actually. In fact one famous Microsoft employee recommended that all Microsoft engineers read it.
But really, it's a good book to read for anything interested in Microsoft, or work with Microsoft products.. not just people switching from the Microsoft universe. It really changes these WTF?? moments to AHA! moments, at least for me. And it's definitely not just mindless MS bashing or poorly sourced stuff. The criticisms in the book are very fair, but it's more informational then critical anyway, and even has a little bit a praise.