I also have Sea to Summit stuff sacks but have the roll top silnyon version. They are super light, water tight, and act like a compression sack. They are much lighter than most compression sacks because they don't have all the webbing and adjusters. I use them for sleeping bags and clothes. Basically stuff the stuff in, press out the air, roll the top down, and everything stays compressed.

As IceNine points out aggressive compression of insulated items (down and synthetic insulation) can damage the items by preventing them from retaining their loft. That said unless one is keeping items compressed for long periods of time a compression bag should not be permanently damage the contents.