I read it about two years ago - it was only slightly out of date - and I really enjoyed the high level introduction it gives people. While it may be addressing developers, architects or managers, and doesn't have as much for DBAs, it was still a great place to get a quick intro.
MongoDB (the company) frequently hands out printed copies of this book at Meetups and MongoDB days. Over the last two years I was saddened that while still being a good intro, the technical details were getting out of date.
The great thing about all things open is that you can fork a github repo, make updates and then create a pull request, which is just a fancy way of saying you can make updates yourself and then ask the owner to include them.
And that's what I did. And yesterday, Karl announced the newly updated book is available to all.