Universal concept that should be at home in any campaign. The Emissary class presents a good addition to the forces of any organization in your campaign world, adding a different option to choosing a rogue or bard for the role of intermediary or herald.
The game mechanics are good and balanced; the class is neither too powerful, nor weak. Entry requirements support the concept and include a roleplaying entry allowing the GM to customize the class to his/her world.
The class abilities make this class a traveling herald not to be triffled with, balancing them out between the social and combat ends of the spectrum. The only thing I would watch out for is that a couple of the class abilities grant the Emissary special magic items. As a GM I would have to decide how often these could be replaced if lost and/or damaged, and how balanced they are in regards to similar items. At first glance they seem ok, so it shouldn't be a big problem.
Overall, a really good class to have in your toolbox, especially for social-heavy campaigns, or those featuring various organizations scheeming with one another.
[4 of 5 Stars!]