I'm just your average gamer, so when it comes to in depth balance I won't be able to help or provide deep insight. I love RPGs but I, not even half as knowledgeable as most reviewers are. So please excuse my amateurish review.
I love themed campaigne settings that provide classes and rule modifiers that suit the setting. I'm huge into real world history, mythology, religions, folklore, etc.. So you can imagine how stoked I was when I first discovered this. I love the Robin Hood mythos and highly suggest all check out the Outlaw series by Angus Donald.
I love low fantasy campaigns based off of real world lore, however I was a tad disappointed with this. The classes seem too simplistic and a tad underpowered. I know this book is going for realism but they still felt too limited. The swashbuckling point system helps fill in the gaps in these overly simplistic/limited classes but its still not enough for my taste.
An older 3.5 OGL product that I feel did a better job at the historical fantasy/low magic feel was Legends Of Excalibur by Charles Rice. The book features similar classes that are more fleshed out and feel more complete. I'd rather add the swashbuckling point system to those classes instead.
The magic/mythical classes are interesting yet still feel slightly too simple. That's not to say Im totally disappointed with this book. Overall I enjoyed. Outside of the classes most else feels right and is well done. I really like many of the new combat feats. I'm just a little sad the book didnt include classes/PRCs with a Crusades theme to them. For example the Paladin could've been modified to be more combat focused and renamed a Crusader Knight dedicated to liberating the holy lands. The Cavalier could've been renamed "knight" and new Orders could've been introduced suitable for the setting. Would've liked a traveling Trouvere(sp?) entertainer or Jester. The list go's on, however these are just my opinions/expectations. Others may prefer the classes herein as is.
Even if you don't like the replacement classes this book is worth getting simply for having a concise Robin Hood Pathfinder setting with great roleplaying info/suggestions. I also really like the new feats like Parrying and riposte.
I'll list each class in this book and suggest my personal similar class alernatives found in other products. Just add the swashbuckling system found in this book and I think you'll do just fine.
The Engineer: Dificult to say. This might be ok as is, however it feels rather bland.
The Knave: thief/archer hybrid. I'd rather use PF's Unchained Rogue.
The Man-At-Arms: PF fighter is better.
The Noble: I'd use Green Ronins noble class or A Game Of Thrones OGL noble class instead.
The Priest: I'd rather use Green Ronin's Priest class presented in thier Mythic Vistas Medieval book.
The Yeoman: Legends of excalibur by Charles rice has a IMO better version of this.
The Conjurer: fine as is, however it feels a tad bland.
The Druid: CELTIC DRUIDS presented in Green Ronin's Eternal Rome I feel is better.
This is all my opinion though. Others may feel differently. Even with my disappointment with the classes I still like this book and can recommend it for the fluff/setting alone.