I should probably start with a caveat about myself. I love prestige classes. I especially love ones that have a lot of flavour AND where the mechanics actually do a very good job of reflecting that flavour. I want a character with a Prestige Class to be less powerful in general than a member of the base classes but to be more powerful within their specialty.
Let me start with the good things about the product.
Every prestige class is very, very rich in flavour. While all fit well into Golaron most could fairly easily be moved into other, similar, worlds.
The book is very well organized. Its easy to read, easy to look up things in play. The artwork is often very evocative.
After reading the book I have lots and lots of characters that I want to play. Not quite one for each prestige class but more than 1/2 of them inspired me.
Artwork is very much a matter of taste but I thought all the artwork at least acceptable and really liked some of pictures. Cheesecake factor is reasonably low (I'll leave the reader to decide if that is a good or bad thing :-)).
Some of the characters are of the sort that, ideally, a group or campaign should be built around. For example, I can easily envisage an entire campaign being essentially centered on a RiftWarden (a class devoted to maintaining the boundaries between the planes). While freely mixing prestige classes within the group may be a bit problematic I see this as very much a good thing and a testament to how rich the flavour is.
I most certainly recommend buying the product.
However, there are flaws.
Some of the rules are mechanically unclear. This isn't a huge deal for most games as the GM can just make a ruling but it makes it a little awkward for PFS.
A reviewer pointed out the Winter Witch as an example. Another example would be the tattooed mystic. It gets Familiar Tattoo at level 1. Its not clear how that would combine with an existing familiar (tattooed or not). As a GM I'd certainly rule that it would stack (transforming the other familiar into a tattooed familiar) but that kind of thing should be explicitly specified.
My other concern is that, mechanically, the classes seem uneven in the cost that they are paying for the mechanics that they receive.
For example, the Lantern Bearer pays almost no admission charge (A few skill ranks that a character would likely have) and gets some quite potent abilities in exchange for little cost.
The Green Faith Acolyte, on the other hand, gives up a fair bit (if entered as a druid) and gets nothing at all in return until 3rd level and many of its benefits are purely flavour or GM specific. Interestingly, the mechanics all but force a Shaman or other Druid archetype that gets wild shaping to delay entering the class until they actually get wild shaping. That could have been addressed but only by writing some quite convoluted language.
The above said, those who concentrate on flavour and not so much on raw character power will love this book. And its not as if its really a major concern if a druid or cleric gets slightly less powerful. Although I'd have to do a lot more analysis to be sure my impression is that the more powerful classes may take slight nerfs in order to take a prestige class while the less powerful may get minor power ups. I'm not sure if that is deliberate or not but its certainly something that I can live with.
As an aside, I did find some of the choices as to what is legal in PFS a little questionable from a flavour perspective although a good back story could certainly justify it for lower levels of the Prestige Class. Fortunately, in most cases by the time the character is getting to a high enough level that they really should NOT be hanging around PFS they retire from most scenario play anyway.