I've had a .Mac account for a couple of years and I'm in 2 minds about the iDisk. Here's my take on the plusses and minuses from my perspective...
Plusses
- Good integration with the Finder
- Supports all the file metadata/spotlight comments
- Can autosync to a disk image on your Mac, so you can access files if disconnected from the internet
- Reasonable bandwidth limits on monthly access
- Good for many applications which use it as a backup or sync location
Minuses
- Pretty slow compared to up/downloading to one of my FTP accounts or Amazon S3
- Even the upcoming 20GB limit is not stellar compared to some of the storage deals around now
- The autosync gave me fairly frequent sync conflicts, even though I don't believe I'd made that many file changes from the 3 Macs I was keeping sync'ed
- Several times I'd get into a 'last sync failed' mode, which I couldn't seem to clear, so I'd turn off the autosync and then restart it. This worked OK except that it would have to recreate the local sparseimage copy by downloading all the files
- I always had a nagging doubt in my mind about keeping personal files on my iDisk. I used an encrypted image for a while, but it got to be a nuisance as the whole image would be re-sync'ed every time I accessed the files in the encrypted image
So I'm currently using Amazon S3 and JungleDisk. I've set up some auto backups in JD, and enabled encryption with my own key, so I believe my personal files are as secure as they can be. The speed is pretty decent because of some local caching, and the cost is pretty low unless you go crazy on uploading loads of media.
My best advice is to try the free .Mac trial and check out iDisk for yourself to see if it works for your purposes