I use iDisk mainly to store and send shared links to Audio projects which I upload as archived ZIP files. These range from single song mixes to AiFF files, stems and ProTools sessions etc.
There's also the option with iDisk that colleagues can upload to your public folder.
This method of sending and sharing music data is pretty much the lingua franca of the pro music community.
I like the fact that with iDisk, anyone can download from the shared link - they don't need an iDisk account. However, my understanding is that shared files on Dropbox require both sender and receiver to have a Dropbox account.
YouSendit and similar services work for sending files but they delete the files after a certain period, whereas iDisk is also an online storage solution.
So, at the moment, as slow as iDisk is, it's still the best all round solution for me. Unfortunately, thanks to iCloud, it won't be for much longer.