You will be able to carry a lot if stuff in that bag. It does make a good backrest as long as it is full. Straps make bag pretty stable. I added a couple of bungy cords since I do not have at tour pack rack to secure it to.
I really like mine. I was able to put so much in the bag it was amazing. Couple of suggestions though. When you put it on the seat make sure the plastic connectors are not on the speaker. It rubbed a whole in my speaker grill. Also it is a good backrest as long as it is full.