My players have been using the new classes since the playtest began, and now that the final versions are out with this book they have been using this version of them. There really is no better way to judge whether a product like this is good in my opinion than actually putting the classes and other options in it to use and seeing what they are like in play.
The new classes have been getting use at my table, with almost all of my players at least trying one out. The results so far have been solid.
These classes have proven themselves through gameplay that they can be fun, useful and interesting, while, I have not in fact seen them make the original base classes irrelevant or unappealing.
It is important to note I have not had a chance to see all the classes in play, some I have only read the entries on, but I have seen the following in play: shaman, swashbuckler, bloodrager, investigator, slayer.
There are some flaws with this product, though the main one are some of the feats, either because they are badly written or do nothing more than make other feat options less potent (and without helping with game balance in the process) but over all this book is full of great options to further customize your character, and the new classes are solid.