The pre-order for this includes a downloadable pdf, so I spent yesterday evening reading the whole thing. It captures the feel of the comic very well - there is an emphasis on teamwork, on being small creatures in a big dangerous world, and on making hard choices about what you beleive in.
Oh yeah... quick aside for those not familiar with the comics... mice are like people, they have a little medieval society and talk and stuff. All the other animals are basically just animals but as mice are tiny they are effectively monsters. The mouse guard are the mice who protect all the other mice from these dangers.
I'm not familiar with any of the other Burning Wheel games, so I can't give any comparison to those, but it's certainly a lovely system. The basic way you make tests is simple (roll a pool of six sided dice, 4-6 are successes, you need X successes where X is picked by very following instructions in the skill description). The more elaborate kinds of tests and things you can do with them (teamwork, use of weapons and tools, etc) build on this base in straightforward ways.
There's also a real focus on getting into play fast - there are premade characters and missions in the book to start you off, and every kind of animal you might meet is statted out (even a bear, although I hope never to see a mouse patrol facing one). Every session each player gives their mouse a goal to try and acheive by the end, as well as the mission you have as a team. The GM sets out to put some obstacles in the way of the mission and goals. Things get interesting due to the failure mechanic. When a player fails a test, it isn't just 'do you succeed or fail'. Instead, the GM decides whether your mouse succeeds, but takes a condition (like now it's Tired, or Injured, etc) or fails, but there is a Twist in the story.
Adding a twist to the story allows the GM to bring in entirely new obstacles or sub-plots, and seems like it will make running a session go pretty smoothly while maintaining a sense of unpredictability and excitement.
The other neat thing is that after the mission is done, the players get a formal turn. This is basically the guards' down time, where the players get to finish up their goals, recover from the mission, and stuff like that. There's some limitations on how much you can do and you still have to make tests to get things done. Having this be directed by the players is a cool idea. I like it.
There's some overarching shape to the stories too - the seasons have a very pronounced impact on the day to day lives of the mice and the missions that the guard will need to do. There are a few ways suggested to use these changes to structure your games. All of them look pretty good (e.g. X sessions per season, X weather changes per season, etc). Over this there's an even bigger structure where the life of your patrol will probably come to a close as the characters retire or die.
Getting back to the point about running the game, it seems like it will fit into my preferred mode of 'think up a basic idea about what will happen in five minutes before the session and then run with it'. The characters have a whole lot of flags written in to the character sheet as well - friends, enemies, things they beleive in, things to roleplay each session, things they are good at, personal quirks, etc. These all have mechanical reasons to be incorporated into play, as well, so they'll be sure to be used.
There's plenty more in the rules, but I think it's premature to say more before play. At this stage, it all looks like it will be great.
Lastly, it looks absolutely fantastic. It is filled with David Petersen's art - some from the comics, some new (all stunning) - and printed in full colour. It's been printed through Petersen's publisher, so I expect it will come out the same quality as the comic collection, i.e. significantly more fantastic than a pdf read on a laptop screen.