I've been carrying a bag long enough to have developed a keen sense of what shape and size works, and what just annoys me to pieces.
As long as I know the dimensions of the bag, I can tell just by looking at it (online) whether or not it's something I could use. However, sometimes when I actually receive the bag, I reject it on quality, color, or as 'not worth the expenditure'; so when I have doubts I try to inspect the bags in a local store before I buy them. Sometimes it makes all the difference. Unfortunatley, very few of my bag purchases were sold at any stores around here, so online buying is normally my only recourse.
I looked into renting, but the membership fees are huge, and you have no way of knowing how long you'll have to wait for your favorite bag to come available. It would take so much money away from my bag PURCHASING budget to rent them first.