We often bike to the farmer's market and grocery stores on our mountain bikes carrying backpacks. So far the heaviest mine has been when I arrived home was 12.5 lbs., and that was plenty on my < 105 lb. self! I could definitely feel it as I rode and had to be careful in turns.

I usually end up carrying just as much weight as my 150+ lb. husband, since my backpack is a bit larger. We just installed seatpost-mounted racks on our MTBs (full-suspension Jamis Dakars) and put our Arkel Tailriders atop them, so that should take a lot of the load off our backs. The racks are rated for up to 22 lbs., but I would not attempt to carry that much weight.