The 6th edition book is rather different from 5th edition. First of all, it is much shorter. The order of chapters is completely redone, and a lot of background information from 5th edition is now only available online. This makes it less suitable as a comprehensive style guide compared to APA 5th.
There are some important style changes that will influence writing papers (most notably how to make reference for papers with more than 6 authors, a change that requires referencing software like Endnote and ReferenceManager to re-program part of their core architecture). So everyone going to send papers should adopt this new version (although many journals still ask for APA 5th style!).
A major improvement over the 5th edition is the extensive section on ethics of publishing, which outlines what counts as plagiarism, how much self-plagiarism is allowable and in which circumstances, etc.
All in all, I will use this new edition next to the much more comprehensive 5th edition for the foreseeable future.