I played BW before Mouse Guard and I must say, I like the rule system. I try my best to stick to the rules and try not transfer anything over from BW but I have used the “Let it ride” feature which my players seem happy about. Yes, I know changing rules… Boo 
Anyways, before I get ragged on, I will tell you what Mouse Guard was like to our group.
We used the pre-gen characters for the grain peddler mission and it turned out well. All players are new to the system but I seems to work out so smoothly. In fact it went so well the players got attached to their pre-gen’s and wanted to resume the game with them. Not many systems can you get a pre-made character and have an attachment to them. Either way we did a few tweaks to personalize the character and we leap forth into new missions. I have used some ideas from the comics to form storylines relevant but not exact. Mostly because I want the players to make their own characters soon and there is a good ‘ending’ in store.
We are on our 4th session tonight, players are excited as I am about it. They can feel the build up of tension as we dive closer toward the climax (Midnight’s Attack). Afterwords, or if they were able to thwart it out without a war… we will make new character’s as I know that is another part where the game shines so well.
That is how things are going thus far. I am hoping to lead these players into the world of Burning Wheel Gold sometime, once they feel they are ready to try a different RPG.
-Jase