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I've actually yet to find anyone say anything negative about F-stop bags.
You have not looked hard enough :-)
The prices are INSANE for what the pack is. $80+ for the ICU alone? Someone was kidding that ICU stands for Insanely Costly Unit. You can buy quite a few decent backpacks for that kind of money, not just a somewhat flimsy foam insert with fabric that is not molded to the foam and tends to sag and allow panels to move around.
The construction is good by photo backpack standards, but a picky and experienced backpacker will be disappointed. The straps are not nearly as comfortable as what you can get in an Osprey. The pack squeaks as you walk with it, due to rubbing of fabric against the frame. The weight/volume ratio of the shell is not bad, but not great either. Just hold the shell in one hand and then an Osprey with an equivalent volume in the other and see for yourself. And the Osprey shell will cost you anywhere from $50 to $100 less.
Despite all these nits, if you can afford it the F-stop stuff is truly unique in the world of primarilly photo packs. It carrier a tripod better than anything I have tried and the harness/frame distributes weight properly on your hips. Yes, you can go on a light 2-3 day hike with it if you strap a UL tent and sleeping bag to the outside. But for stints like that you are probably better off with a proper backpack and a Mountainsmith Cube insert.