Not everything benefits that much from compression, cotton for example. Fleece and merino wool, however, are really compressible. I have two Medium REI sacks, the green ones, not waterproof, and they seem tough enough. I used them a few times and I don't see how they can tear easily unless they come in contact with something sharp. Their quality seems to be similar to Sea To Summit sacks that I own.

I have second thoughts about compressing my down sleeping bag though. I think it really crushes the feathers and I imagine that can can't be good.

As someone mentioned above if you put several compression sacks in one pannier there will be empty unused space between them. So what I do I only use one sack per rear pannier and put it on the very bottom of the pannier. I use them mainly for emergency warm clothing, such as the mentioned fleece and wool, and sleeping gear that I'm unlikely to need regularly during the day (I don't tour in cold weather).

Then I have a couple of waterproof, large Sea To Summit compression sacks that I used a couple times basically as an additional storage attached with a net to my rack, a rack pack so to speak. They are watertight actually, they will keep their contents dry even if submerged in water.

For example, I used the watertight sack for my sleeping gear plus my sleeping clothes and any items I might need at the end of the day, all in one sack. So after pitching the tent, even on a wet day, I would wipe the sack throw it inside the tent and have all my sleeping needs dry and in one place.

Things I have that benefit from compression sacks:

- pillow
- thermal liner for the sleeping bag
- small thermal blanket
- fleece and wool top(s)
- merino wool items that I use as backup and some as my sleeping clothes, they're very cozy and comfy
- most bike clothing and socks can also be compressed together in one sack

Lots of small, soft items like socks, bike shorts, swimming shorts, jerseys, t-shirts also benefit from being all in one compression sack. I also prefer to pull one sack out of the pannier and find items in it, rather than digging through a pannier that is hanging on the bike. But I bet this could be a personal preference and vary among people.

I'm still experimenting with compression sacks though. They do help, but you can't use too many and they don't work with everything. Keep in mind that some things, foam in particular, may not tolerate crushing very well. And stiff, hard items will lower the effectiveness of a compression sack and might contribute to damaging it.

Adam