We've got four of them, (one the biggest size) and we only use the flip part if the ground is wet or dirty, but it works fine, the very bottom of the bag (nearest the waist belt) is the only slightly akward bit as the zips stop just short of the bottom, but it's not a problem, I've got mine set up with a body and lens in the bottom centre and I have no problems getting it out easily enough.
Mines been out in really heavy rain and the built in rain cover kept it dry inside (sadly it didn't keep me dry as well) the bags also been in some rough stuff and seems fine, they are pretty well made and tough, the only issue we've had was one of the guys lost the chest strap thingy when it got caught and he pulled it off the slider/ajuster but it was easly fixed after he found it.
All in all a good bag.