I also LOVE space bags! last christmas wanted to send an afghan i'd crocheted to my dil; it was huge...puffy...i stuffed it into a space bag, vacuumed it and made it fit into a flat rate box...she about died laughing when she opened it up...they were amazed i managed to stuff such a large item into such a small bag. i have not had problems with them not holding. bought them at bed, bath & beyond for about $12 for a box of 5 different sizes.
i have some bedding that never gets used stuffed into a large one in the linen closet that has been there over a year, still flat and solid, no air leaks.