Coming into this game having played the first two games of the Risen series, I was not disappointed. The game, like the previous ones possessed a certain je ne sais quoi, that made it quite captivating and had me sucked in from beginning to end.
Part of the obvious appeal is the open world environment, and with the ability to make various decisions relating to the relative "goodness" of your character and the faction you align yourself with, it creates a multifaceted game world that helps to immerse the player into the story. Though the storyline itself is not entirely complex, a bit one directional, and has certain aspects of it that might leave someone who had not played the first two games scratching their heads wondering about back story, for those who are familiar with the previous two games, it provides a sufficient thrill.
As for gameplay, the controls (at least on PC) are easy to pickup and make for a workable interface, the skills system (while admittedly not my favorite) is still fairly simple, and the character and inventory screens seem to follow what you'd expect from such a game. Though the enemies you face in-game are not quite as varied as I could have hoped, and some more boss battles wouldn't have hurt, overall it's not too disappointing. One aspect the game could have done without however, is the sea battle sequences, they seemed to be a bit disjointed and redundant, I wouldn't have minded more open world sea battles had it been done in such a way, but for them to be forced sequences kinda ruins the illusion of a truly open world and the facade of choice within the story.
Long story short, if you played the other games and liked them, play this one too; If you played the first two and hated them, don't play this one. If by chance however, this is the first Risen game you've played, while I could see it going down as mediocre, you should definitely give the other two a play first before making up your mind.