Mouse Guard is not the usual RPG. If you are a d20 fan you may even not like this system. That would be a shame, because Mouse Guard can make a great storytelling system.
The core system is derived from Burning Wheel. Key to every player character is their goals and beliefs. Even missions have player-character created goals. This part can be hard to grasp for many players OR GMs. Good goals and beliefs that are played well will result in an enjoyable game; ergo poor goals or beliefs played poorly or with little understanding will not yield a joyful experience. You really need and EXPERIENCED GM to make this system work; having a GM that leads the PCs through the adventure is not enough.
The turn mechanic (GM turn, player turns) seem more like storytelling than roleplaying at times. Again, a good GM will be key.
But once you get through goals and beliefs, and play a few turns it all fits together. I have played this game with my kids (ages 14 & 11) and they really enjoy the adventures. Our first mission was a simple "Deliver the Mail" that forced them to:
1) Find the trail after the long winter (Use your skills)
2) Face an animal threat along the way (Learn combat)
3) Reach their destination, only to learn of a mystery (Social Conflict rules)
4) Search for the source of the mystery (Creative use of use your skills)
5) Face the villian (Combat or Persuasion?)
Mouse Guard deserves a place in every RPG collection.
So why not 5 stars? The pdf is derived from the 8"x8" book print, meaning lots of empty space on every page. And each page has a background. So if you want to print you are going to have to use LOTS of ink and have LOTS of white space around the edges.
The solution...BUY THE HARDBACK COPY TOO!!