I wasn't sure what to expect with this purchase, as I'd used other compression-type products for travel without much success, and was somewhat skeptical of the positive reviews of the Eagle Creek version.  However, I'm really glad I took the chance and bought them, as they worked quite well overall.  The smallest of the 3 sacks is really pretty tiny; I used it to pack more underwear than I would have otherwise brought (hey, you can't have too many pairs!), and it would be useful for keeping tiny things together, but it's too small for most clothing in general.  Since my suitcase was 21" long, I found the medium bag the most useful, and I was actually able to pack eleven summer tops into it (camis and lightweight long-sleeved shirts).  The largest sack was too big for me to take on this trip, but it will be perfect for packing bigger items when I take my larger bag--or for storing items like a down-filled parka or comforter.  It's amazing how small these sacks compress to, and how much they will hold!  If you haven't used this type of product before, you'll want to know that they do not compress completely flat, and you'll have a lumpy, awkward, and relatively inflexible item to work with once you've removed the air from the bag.  Also, it will weigh more than you expect it to, (which makes sense, since you're compressing things into a smaller surface area but they still have the same weight), which means that packing may be a bit more challenging...AND you may be tempted to overload your bag and run into problems at the airport!  Overall, though, they're very helpful, and I'll probably buy more of the medium size to use on my next trip.