While I have no direct experience with panel loading types, I just keep imagining a busted zipper that won't let me close up the bag and I'm screwed. It also doesn't seem like you'd really be able to get at the stuff at the bottom without either risking unzipping it while on the bike and everything dumping out, or taking it off the bike to open it up. But I may just be a bit paranoid.

I've always used organizer pouches. Zipper, drawstring, velcro. That allows me to sort through the bags and instead of looking for one small item, I'm looking for one particular colored pouch.