I use it for poseidon itself so I can boot from USB massstorage devices.
I also have The Os3.9 rom updates in it, a NoIDE module to disable the A4000 IDE and the Morelibspace module from AFA-OS.
If you have a Xsurf, you can put XsurfIDE in it, so you can boot from harddisks attached to te Xsurfs IDE connector.