Today my daily backup fired off and crashed with the message that my iDisk was full, which is un-possible because I don't put anything there. Attempts to find out what were filling the disk failed; it was so slow that I couldn't even find large files or directory sizes on it without timeouts. Meanwhile the attempt made my entire system grind to a halt; other applications stopped receiving clicks, the screen began to update slowly, the whole overload drill. The logs got spammed up with diskarbitrationd explaining in detail that each application was not responding.
I finally managed to unmount the sonofabitch and ran backup again, and manually removed all my backups. Once again everything ground to a halt, no progress bar, nothing, and I lived by faith as the LORD commands. After about fifteen minutes it suddenly finished and there was all my disk space back.
The backup doesn't clean up after itself and I see no option to fine tune that. I'm not paying annual rent on something less configurable than a product I can buy. Foo on them.
So that's it for dot mac.