gyp23 wrote:Tell me, are your bags waterproof? Will the contents get wet if its raining (hard?)
No they're not, and yes they will, but having said that, they're pretty good. They do a pretty good job of keeping 95% of the water out, but when the going is torrential the last 5% gets in

. My Ventura was a rectangular jobbie with a zip around the top which gave you an extra 15-odd litres when you needed it. Pretty handy and held shit-loads.
A tip: I bought some waterproof bags from Mountain Designs (intended for Kayaking) and used to put my Walkman/wallet/mobile in them when it was inclement. They keep the 5% from getting into stuff you don't want to get wet. Buy the ones that seal by folding over three or four times and clip shut, not the heavy-duty ziplock ones, as they are way cheaper and come in a multitude of sizes.
Grocery shopping on a bike is going to be a pain unless you like shopping every couple of days. Have you considered investigating the e-grocery shopping via the web concept? I recall seeing that as an option in Sydney, may be cheaper and quicker in the long run.
A Givi top box is a better option, more secure and 100% waterproof unless you ride into a creek. However they do cost more, and the big ones capable of holding the same as a Ventura pack make you look like a gumby.
gyp23 wrote:am I right in understanding then that the Ventura has a dismantable rack system, whereas the GearSack doesnt?
I can't vouch for the Gearsack system, but the Ventura one is removable...to a point. The 'L' brackets stay behind when the back comes off. Pretty ugly.
HTH.