Like others who've already replied, I also have a Timbuktu (med) bag and love it; it's durable, handy and well-made. That being said, I rarely use it for commuting any longer as I quickly found that I don't like the (usually shifting) weight on my bike and prefer to use a rear rack instead. I am the same way on long rides - centuries or double centuries - and only use a small (70oz) camelback that barely reaches my mid-back and doesn't bother me as an unnecessary weight.
It's all in what you prefer, but also, if you use the stabilizer strap on the Timbuktu, it takes any shifting out of the equation, so that was never an issue for me. Just found that I prefer to use a bigger bag on my rear rack when necessary.
Mary