We stayed in Twin Lakes December 2008 between Christmas and New Years and it was never really busy. Rode to Lake of the Clouds one day and Copper Harbor another, and some other trips in the area the other two days we were there. We would occasionally come across a large group but there were never lines at gas stations, backups at trailheads, etc.
I've ridden all over the UP and in the past 4-5 years I have never experienced a 'busy' week or weekend compared to 2000-2004 years when it seemed like all the hotels were full, the gas lines were long and the trails crowded. Seems like a lot less people riding.....