I GM'd Mouse Guard for my old Seattle RPG group this past weekend. On the whole it was a great success. It took about an hour for all of us to get into it, and understand the rules.
When GMing, the rules tell you to find two challenges for the party, and just go for it. What they don't mention is what if the party succeeds? On failures, you can add all sorts of fun twists, but successes just succeed. Keeping the GM turn rolling for two hours could require a lot more than two encounters.
I used the sample mission "Find the Grain Merchant," and the sample characters. I used two others from other missions as I had six players. I added a lot more complications, as the game would have been over (the GM part) in about 30 minutes otherwise.
The Player Turn was interesting. Using the examples in the book, I could see that the players could keep the adventure going by spending points, but I'm still unsure how many points they would need to go and get into trouble and find the conspirators. The rules are very clear on how to spend points to heal. They need work on how the players can go off and create story.
All six players enjoyed the game and wanted to play more. Always a good sign. I suspect a rereading of the rules and some searches on forums will be in order before I play again.