I’m running Debian/Unstable on an EeePC 701, I’ve got an SD card for /home etc but the root filesystem is on the internal 4G flash storage which doesn’t have much spare space (I’ve got a full software development environment, GCC, debuggers, etc as well as running KDE4). On some of my systems I’ve started the practice of having two root filesystem installs, modern disks are big enough that usually it’s difficult to use all the space, and even if you do use most of the space the use of a second root filesystem only takes a fraction of a percent of the available space.
Today I discovered a problem with my EeePC, I had upgraded to the latest Unstable packages a few days ago and now when I run X programs the screen flickers really badly every time it’s updated. Pressing a key in a terminal window makes the screen shake, watching a video with mplayer makes it shake constantly to such a degree that it’s not usable. If that problem occurred on a system with a second root filesystem I could upgrade the other a few packages at a time to try and discover the root cause. But without the space for a second root filesystem this isn’t an option.
I hope that Btrfs  becomes ready for serious use soon, it seems that the btrfs snapshot facility might make it possible for me to preserve the old version in a bootable form before upgrading my EeePC (although even then disk space would be tight).
So I guess I now need to test different versions of the X related packages in a chroot environment to track this bug down. Sigh.