Here's my Eberlestock Halftrack next to a friends Alice pack:
I don't always run the pack this heavy, and in this image I have a lot of "extras" strapped to the outside since this was a setup I was testing for a 40h game. On the bottom I have a USGI bivy, on the back is an Adapt pack, and a GP pouch below that. On the right side is some condor pouch with my jetboil. I also have two EI-style zippered med pouches on either side of the waist belt to hold tools and quick-access items I need while wearing the pack.
I built this setup with the idea that I can drop the main pack and detach the adapt pack, and still have all of the essentials with me (water, spare mags, some food etc...); although I find myself doing this less and less. My goal is to eventually grab a single, larger pack (probably mystery ranch) that in theory should carry and distribute larger loads more efficiently. I've found the halftrack to be somewhat of a "squat" pack, and given the way I have it set up in this image, I feel loading it up on the outside of the pack causes it to hinge away from my body and put extra pressure on my lower back; all the extra pouches = extra weight the pack wasn't designed for.
All said and done this setup weighed about 45lbs with my water and wet-weather gear (a bit more once I strapped a spare gun to the outside). This is the heaviest I would ever go with this pack, or probably any pack for that matter.
So... think about how much capacity you need, and definitely consider the dry-weight of whatever packs you are looking at.