I was dubious when I bought these, as questioned whether could really work very well with just manually squeezing out the air.  But worked like a charm!  I used it to compress a twin size air mattress in the "large" size which easily reduced the size by over half.  Also used the "medium" size for a queen-size blanket which also worked wonderfully.  Highly recommend these if you want to squeeze more stuff into a smaller suitcase.