I suppose you mean .Mac ("dot-Mac").
In general I have a problem edit: (concerned, really) /edit: from a safety standpoint when backing up data over the internet.....
I do use it for trivial stuff, like bookmarks, Addressbook.app data and so on. But if it were REAL important (and senstitive) data, I would NEVER back it up to a medium which is not physically within the reach of my own two hands.
But I haven't heard of any "oops" stories regarding .Mac