We bought Space Bags years ago and they still work very well for us. They are the kind that you suck the air out with a vaccum cleaner. It does take a few trys to make sure that you seal/zip them up correctly, or they won't work. My mom bought some not long ago and didn't have luck with them, but I have a feeling that she wasn't doing it right. I would keep in mind though now that you may not want to use them in your suitcases that you take with you on the plane for a couple reasons: first is that the max weight for most flights now is only 50lbs and when you use the bags, it tends to make things quite a bit heavier because you can fit more in! Second is security in the airports. I have had many reports from friends that have just been flying domesticaly that they are checking a lot more bags than usual, and if they have to search your bag and open it, there is no way they will get all of the items back in!
I would recommend them if you get the good ones that you suck the air out with the vaccum, but there are cheapo copy cats out there that don't work nearly as well.