What do the SIGforum outdoorsmen have to say on the subject? Until yesterday, I had experience with Alice packs and the current Army rucksack; the only internal frame packs I had used were small to medium sized. I borrowed a USMC ILBE ruck from a friend yesterday, and found it to be quite comfortable. A few days prior, I had toted my Alice "bug-out bag" around a bit, so I had a recent experience with which to draw comparisons. My packed Alice is fifty pounds, and has served me well enough, comfort-wise; I reckon you can only expect to be so comfortable with that much strapped to your back. I stuffed the USMC pack with two cases of 5.56 ammo and a few quilts, in an effort to put some weight in there, but also fill it up, to try it out. Ended up being seventy pounds, and I feel like it carried better than the fifty-pound Alice. I am 6'1" and 165LBs, to give an idea of my body type. I understand the near-universal complaint against internal frames is the lack of airflow between the pack and the body, and I certainly came out of my walk with a sweatier back than I would have with the Alice. I think that con is outweighed by the pro of more overall comfort otherwise though, for me anyway. A drawback I do recognize is the inability to wear any type of load bearing equipment on the torso; any clips or friction sliders would be inevitably ground into your back. What are other members' experiences, and what packs do you prefer?

I'm no expert but my Osprey 45l pack has this mesh 'suspension' that keeps the body of the pack off my back. Super comfortable. I seem to recall the Gregory brand models having something similar. I would bet the USMC packs are built more simply, since Marines can be counted on to break the most bombproof of things. Their packs probably lack a lot of the nicer things a civvy pack offers.

Internal frame packs come out of the alpine and ski touring world, where the need to keep your center-of-gravity close and maintain a fair-to-high level of movement is necessary. The inherent nature of the body hugging design and increased padding make them very comfortable, particularly in uneven or, rough terrain. Frame packs are as old as man himself, you can carry all sorts of awkward and odd sized items, hence that's why they're very suitable for hunters or, a mortar team. If you're on fairly even ground or the path you're covering is established, a frame pack can be just as comfortable as an internal frame. The old ALICE style of packs works on a number of levels and is well suited for the Joe's in the mud, its not the most comfortable or, best constructed pack but, its simple, uncomplicated and works for its intended usage.

The ILBE you got was designed and made by Arc'Teryx; a number of SEALs and USMC Mountain Warfare instructors pushed to get them into the issue pipeline; they're derived from their established Bora packs. The problem with internal packs is they require fitting. Two people need to spend the time making sure the frame stays (the internal stiffeners) are shapped correctly and the straps/pad-settings are put in the right place. If I recall, there was an instructional booklet and CD-disk that came with each of the new packs; not only did each Joe look at each other in confusion but, their platoon sergeants freaked out. Needelesstoday, while the USMC got a VERY nice backpack, it was completely lost on most since many never took the time or, understood the necessity for correct sizing/fitting.

On the commercial side or, open market, internal frame packs are hugely dominant because of the comfort factor. Carrying 50-60#'s sucks regardless of pack-type but, if you can make it manageable, than you'll win converts. Ventilation/over-heating will always be a problem given the inherent nature of the packs, hence that's why they're better suited for the higher elevations versus below the treeline / basic backpacking.

Osprey is the dominant brand in this category, they own about 30-35% market share in just backpacks alone and nearly 50% in technical packs. Dueter has become a major player due to their more appealing price-points and they seem to suit women's frames a bit better than others. Gregory used to be pretty big, their market share has shrunken lately, their massive Denali packs are the go-to for folks looking for big haulers. Mystery Ranch is Dana Gleason's company up in Montana; Gleason was the namesake of the iconic brand Dana Designs then recognized as one of the foremost innovators of internal frame packs. MR has established themselves within hunting, firefighting and military circles with a range of well designed packs, many of which are USA made. Downside is Dana's packs have always been on the weighty side of the equation. For more military oriented packs, Kifaru has a very strong following, owner Patrick Smith is an interesting guy with some strong opinions.

The USMC packs are designed by Arc'teryx, and built by Propper. They are pretty nice, albeit a tad heavy when compared to other contemporary packs. And they're American-made, which is cool.

Well, corsair and I posted almost simultaneously. Thank you, sir, for the good info.