I have used several different backpacks in the field with great success. Packs are like boots, buy quality up front or you will quickly regret it.
I own an Eberlestock FAC Track and highly recommend it. Its got molle on the inside and out so you can organize the pack interior using molle pouches. Its tough as nails, roomy, but not too big or bulky. The zippers go all the way down the length of the pack so you can lay it down, open the flap all the way, and pretty much see and access all the contents in the main compartment.
I was issued a Mystery Ranch Divide pack for a deployment to Afghanistan. That thing was a hand me down (got it from the guy I replaced and I passed it onto the guy who replaced me). Abused, beat to hell, and was still going strong. I like all the separate compartments so I could section off my gear and tools. It made it easy and quick and I didn't have to dig around too long for things. Wish I could have kept it, but had to pass it on. Can't justify purchasing one since I already have a few high dollar packs.
Blackhawk 3-day pack. I have the one without any molle on the exterior. I wanted something not too tactical looking. I pretty much use it for a range bag now, but used it in the field as a gear hauler in the past. Hasn't let me down yet. It too has not been treated gently and still going strong.
If you can get your hands on a surplus Army Medium Ruck, those things are great too.
