The weight can really make the shoulder ache after few hours...lol.
Nowadays I don't even bring the 8mdh for overseas trip, always end up with a half filled bag only...plus too small to store all my carry on belongings, yet too big to fit under the airplane seat as a bag that sticks beside me and carry all my impt items.
I have the 8mdh and 7mdh and the Thinktank retro 10,20,30. If you do make use of all the space in the 8mdh, it's really heavy, esp if you put a laptop inside. I feel it's more for transporting gear and if you have transport like a car or bus. So, it's just carrying for short distances before you can rest and put it down. It's quite a big bag and looks big on a 1.7m tall person. Crumpler bags are heavier than the TT retro bags of similar size because of the material, padding etc. The shoulder padding on the TT retro bags are one of the best. For similar weighted equipment, prob can carry longer period with the TT retro bags.
Sometimes, it's not the size of the bag. I also have the urge to bring out all my lenses, but sometimes it's not practical if you are walking long distances. If you are physically fit, then I suppose it's not a problem with a fully loaded 8mdh. For me, body+2-3lenses+/- lens is about the max I can carry for a whole day out