It is incredibly buggy. Some days I feel like I'm wrestling with a giant squid that is wrapping its tentacles around me sucking me down and as I valiantly hack off its arms they grow back, their destructive suckers adhering to my carefully chosen words and formatting and distorting, maiming, killing...
The really evil thing is that Word has the ability to learn. That is, it learns to stop destroying my doc in one way and then learns to destroy it in a new way.
For a while it did this: Whenever I formatted a paragraph, Word applied the formatting to every paragraph in my book. If I made a paragraph a bullet, every paragraph became a bullet. Then I would press Ctrl-Z to undo and Word would roll back the changes to all paragraphs but the one I wanted to change. This went on for a month or so, then it stopped.
Now it is targeting my internal cross-references. I created hyperlinks to headings within my book. A link will all of a sudden be corrupted in the most pernicious way. Several weeks ago I cut and pasted a section of text by highlighting it and pressing Ctrl-X. Although I have put thousands of things on my clipboard since then, Word remembers that section of text and it inserts it into the middle of a hyperlink. It does it to links that I haven't even changed recently. The phantom text just appears. I created headers for my chapters, and Word particularly likes to insert the phantom text into the headers of certain pages in a chapter. At first it was extremely difficult to delete the phantom text. Then the monster "learned" that I wanted to delete it and made it possible to do so, but it keeps inserting the text!
I also own a copy of Acrobat and I use it to create PDFs. I'm not sure whether it's Acrobat or Word that is the culprit, but again inexplicable bugs are cropping up. Like links are getting produced with internal numbering revealed.
I work responsibly. I pay for licensed copies of software. I put up with the chaos of automatic updates in order to get bug fixes in a timely manner. I read the Word forums to keep up with the experts on best practices.
I can always find a workaround. It takes time and it's annoying, but I can find a way to produce a decent book even with the insane bugs. But why oh why is Word so buggy? Why can't the world's largest software company make its flagship office product work reliably? Why does the new Word interface emphasize little-used bells and whistles and make key functionality more difficult to use? Why oh why???