I am a great fan of the Sea to Summit bags. Mine range from small to large, and I use them in colour-code to separate various items. They are cheap, which is a great plus.

The material is highly pliable but remarkably tough, and I can heavily compress-stuff my sleeping bags and clothes into them, something that is less likely with trash compactor bags. When I have used those types of bags, the biggest problems have been getting the air out, and then sealing off the top so the compressed stuff inside doesn't "reinflate" the bag.

I have had a set of S2S bags for around four years, and they have done a power of touring and randonnee work, and while they each have shown some moderate wear, they have stood up well.

I am somewhat paranoid about getting clothes and sleeping bags wet. I use a small one for my tools, and that loops and clips over the rails on my Brooks saddle. I figure they have outlasted a pack of trash-compactor bags that have that age old problem that goes with panniers and roll-top bags -- the thing you want most is always under everything else. Plus, I am not so inclined to dump waste plastic that often.