I Really Like aufs...

Being able to overlay a tmpfs on top of a read only file-system is really handy for embedded systems, particularly ones where a read-only device (e.g. ROM) or a limited-write-cycle device (e.g. flash) is used for the root file-system.  To programs running on the device, the file-system appears-and-acts writable, but files created, modified, or removed are done so in a RAM-based tmpfs instead of being persisted to the backing storage device.  Not only does this help increase device write lifetime (in the case of flash), it can also improve device stability in the field.

Certainly there's a number of "union" file-systems out there, but I've had good experiences (and stability) with aufs.