For starters the artwork is simply beautiful. I don't think I can remember being this affected by comic artwork since the first time I saw Mike Mignola's Hellboy. Like Hellboy, Mouse Guard has a visual style all of its own and is also written and drawn by the same person, David Petersen.
A lot like the fantasy series Redwall, Mouse Guard follows little medieval mice as they try to survive and protect their own lands. We follow a small group of mice, members of the Mouse Guard whose job it is to protect their lands and cities. The first volume follows the actions of three specific mice Saxon, Kenzie and Lieam who discover a possible treasonous plot that may lead to war.
I've got to say that some of the most amazing things about this series for me are the action sequences. In perhaps the most striking one in the first volume two mice battle giant (to the mice anyway) crabs (pictured right).
If you like medieval fantasy, or simply amazing artwork and story, I would definitely try out Mouse Guard, it is simply amazing.