I'm generally of the opinion that panel loaders suck, but I have to say that I've had a Lowe Contour Mtn 40 pack for 10yrs now and I'm really impressed with it. The zipper blew out ~5yrs ago and Lowe warranteed it w/ a really beefy zipper that has been going strong ever since. The straps and bottom have actually been disintegrating lately, so it looks like the zipper is going to outlast the pack.
I think the secret to this pack's success is that the zipper only extends 1/3 of the way down the pack, so most of the load is exerting force on the fabric, not on the zipper. Of course this makes the pack handle a lot more like a top-loader... Another good design feature of the pack is it's tear-drop shape - the back-to-front dimension is much larger at the bottom of the pack than the top. This again keeps the load off the zipper and keeps the weight in the small of your back.
And although everyone else thinks shovel-flaps are uncool these days, I like them for quick stashing of clothes and even an occasional shovel... I don't want to go digging through my pack for my shovel, losing all of it's contents, while my buddy's trapped in an avalanche.
Just my $.02