No idea how many people. But earlier, ReiserFS was quite common on Linux. And I have a lot of USB disks still formated with ReiserFS.
Also, there is still no really good cross-platform filesystem (at least for Linux + Mac). HFS+ doesn't work with journaling on Linux. ext3 is read-only on Mac (afaik; has that changed? not sure). FAT is too primitive and also does not have journaling. NTFS is too less Unix-like. And ReiserFS was not supported at all under Mac. Now, at least it can be read. (I also thought about porting over the full think. But that is much much harder.)
I'm not sure about other file systems. But to my knowledge, there is no filesystem with journaling which can be read and written by both Mac and Linux.
On Linux you can read most (all?) of the file systems FUSE supports already.
I don't really understand the MaxOSX use-case. Do many people really want to mount ReiserFS on OSX?
ZFS support is nice, but I'd imagine that http://code.google.com/p/maczfs/ is a better route to that.