well, i just got issued my small pack and being a reserve sig out west, i find the thing great, took it with me to quebec for a weekend long course, did the job fine.
rarely do i see winter conditions, so no worries on the buckles, rarely am i dismounted, no worries about loading awkward items, it's really a wonderfully designed piece of kit for me. i have the CP valise i attach to the top, sadly i can't attach a cot or recce tent easily to it as i did my ruck, so i run the risk of having sombody run off with my coveted sleeping kit now, but oh well.
for a cbt arms trade like infantry, i can see the issue of having no easy way to attach it loaded to a ruck, save perhaps some aftermarket straps hooking into where the detachable wasteband on the small pack attachs to. then perhaps using some sorta scrounged strap to attach the top to your ruck.. however it's definatly gonna leave you very bottom heavy. maybe moving your valise to the top of your ruck, and putting your SP sidways in place of the valise at the bottom of the ruck and attaching your air matress somwhere clear on the back.
*shrug* it certainly is exactly what i need for my occupation, just like the tac vest was, sombody at CTS really likes us sigs. the SP even fits my radio better.