I've been using mobileme since they created mobileme and I was a big idisk fan till I actually tried dropbox about a month or so ago. Sorry, but it blows idisk away.
The only thing that kept me using idisk is I paid for it so I didn't completely phase it out.
As far as security. No system is perfect, and even on my own dedicated servers anything remotely sensitive is encrypted. Sensitive data and online don't go in the same sentence with me.