i loved my medium one, with no frame, og green, didn't realise how much until it was gone. now have a large pack, have to have a frame for that one, very good also, use it for the fishing gear, mine came from 22's stores at hereford but nothing special about it apart from the sand in it!
i have to say, i do prefer the medium pack dimensions, and have thought about having a padded back fitted by a kit modder to make it a soft carry job, thought about having the snugpak rocket pak back and straps added.
they are bomb proof as you'd expect from military, they aren't waterproof though, we're talking jungle warfare/desert equipment here, drainage holes in the bottom and only the lid has a water proof backing on it which decays if abused.
someone said earlier a marmite thing, i love them, but be aware, they do creak with age and heavy loads.
i did get a GSbergen which is similar to the medium alice pack, doesn't take as much, and the fframe being steel and not ali as in the alice packs case weighs a tonne!
i'd give 'em a go. plenty of og ones available. fairly cheap. last forever. cult status.