Warm and Natural is really nice, but too expensive. I bought a huge roll of Hobbs Heirloom 80/20 for a lot less, and it's almost as good (IMHO). Works fine for quilting on my frame.
If anyone is interested, I bought it from here:
http://www.heartbeatquilting.com/battingmuslin.html
Edit: they also appear to carry Warm & Natural -- $178 for 124"x30 yards or $172 for 90" x 40 yards. You have to pay shipping, though, but it's still probably cheaper than Joann.