We use them, but also found that the 2 1/2 gallon ziplocks work just as well.
For the kids I packed a days worth of clothes in the gallon zip locks, and squeezed the air out, sealed them, and then each day all they had to do was grab a bag. There clothes weren't wrinkled even after several days like that.
One word of warning though. The clothes still weigh the same even if they take up less space, so if your suitcase is big like mine, I couldn't fill it up with clothes all packaged that way or it would have weighed too much for the airport.