I used the Helinox Zero - once - on an overnight backpack. I found it comfortable, stable, easy to assemble and disassemble, and reasonably easy to get in and out of. It was just more weight than I was willing to carry. "Than I" is the critical part there: at about a pound and a quarter with the mat that keeps the legs from sinking into soft ground (i.e., everywhere I camp), it's not objectively heavy - just more than I wanted to carry. It's a good product, if you are OK with the weight. (Kind of like the Svea stove - a great product, IF you are OK with the weight. Strictly personal decision.)

I'm also experimenting with using my pack as a chair back. My Osprey Levity 45 (like the Exos it's a cousin to) is actually an external frame pack (yes: perimeter suspension with a tight backband, just like my 1980 Camp Trails Adjustable II. However, it uses arrow shafts instead of electric conduit, and a sleeve instead of grommets and clevis pins - huge improvements!) I used to use my external frame pack, propped up by my hiking staff, as a chair to lean against. I'm trying to figure out whether the Levity can take the weight applied "against the grain" (so to speak) without bending. (Electric conduit worked great!) My first trials found it to be a bit tippy; I'll let you know if I get it figured out. By the way, I can't claim credit for the idea: yep, Colin Fletcher, in the original Complete Walker (1968 or thereabouts.)