I got one of these after seeing it mentioned on a top 10 backpackers pillow list. It is designed to perfectly fit the head area of a mummy bag, but I use it with my rectangular sleeping bag. It compresses well (fits into a Sea to Summit Thermolite Liner stuff sack) but doesn't go limp or flat easily like most of the other backpacking pillows out there. I tried a bunch of different pillows at an REI outlet, but most of them just went pretty flat once my head was resting on them. "This isn't what a pillow's supposed to act like." I thought. (I use a 4-5 lbs rated, thick memory foam pillow at home)

After getting my first Sleep EZ pillow I tried sleeping with it a couple of times before ordering an additional one to stuff into the pillow case with the first. That's my current setup and I think it's the perfect combination between packability (both stuff into the stuff sack that comes with either one) and height. One pillow alone is good - great if you're sleeping on a thick mattress (that has some give) or perhaps a hammock. But 2 put together make just the right height to sleep comfortably on ones side. I have slightly broad shoulders and with the setup of two of these in the same case together (not stuff sack, as each pillow comes with it's own cover) it creates just the perfect height required for sleeping on ones side.

Buy 1 if you never sleep on your side and just need a really comfortable pillow that compresses to the size of two fists put together.
Buy 2 if you, like me, desparately need the extra (height) support for sleeping on your side.

It took a while to fit the two pillows in the same case, but, like I said, they're compressible, and eventually I got the first one in with the second (after popping a single thread on the pillow case, admittedly) and the result was, to me the perfect backpacking pillow.