Being a glutton for punishment, I thought I'd try and use Ubuntu One for firefox bookmarks.
I'm not going to trust it with files any time soon, but how wrong can bookmarks go?
First, I had to unbork the completely screwed setup on my laptop. Here's how I did it:
First, turn it off, and then make a backup copies of existing synchronised things on all machines. Just in case.
Then, go to your Ubuntu One account, and cut off your machine from the central sync server.
If you're paranoid, you could even disconnect from the network at this point.
Once you've got your local setup cornered and alone, it can do no further damage to the central files.
The trick now is to hunt down the debris it has left all over your home directory, and kill it all:
$ sudo rm -rf ~/.local/share/ubuntuone
$ rm -rf ~/.cache/ubuntuone
$ rm -rf ~/.config/ubuntuone
Then under Application/Accessories/Passwords and Encryption Keys, find and destroy the UbuntuOne token.
That should put you back into the default state.
Epilogue. That seemed to work a treat. But no bookmark synchronization. Apparently it's broken. Bloody shed.