I used a Bob for the TransAmerica last summer, so it was with a racing bike, not a mountain bike. It worked out great!! Definitely cuts down on wind resistant...the reason I believe this is because we toured with some friends who have panniers and the differences in how we rode were remarkable. You don't need fancy stuff sacks to store things in, just buy some plastic Ziploc bags and suck all the air out before you seal them...this really cuts down on space in the Bob bag. I can't compare bob to panniers because I've never had them and probably will not have them until I get a different bike...my bike probably wouldn't appreciate it.