Mail - Yes, followed by Yahoo, and Gmail to a lesser extent.
iDisk - Yes, daily.
Calendar Sync - Yes.
Contacts Sync - Yes.
iPhone finder- Not yet (knock on wood)
My wife and I both use iDisk w/ sync. She's a teacher and needs access to her files when her computer is off or otherwise not available, so it works out really well for her. For me it works well and it has been MUCH better than it was this time last year--it was a total fail after the iPhone 3G was released, but apparently SJ fixed most of it. Other than hiccups sending e-mail (SMTP) at times, it seems to work pretty well for the most part.
As far as iDisk, I use it a lot for document sharing between my iMac and MBP, "sharing" (class projects), and it's already around 2GB and growing.
The only reason I haven't really jumped ship to something like Zumodrive or Dropbox is b/c I'm rather distrusting of some of these up-and-comings, would rather take a chance on a more established company (ie. Apple, Microsoft, etc.) for reliability (and privacy).
For instance, I used to use Skydrive (skydrive.live.com) when it was in beta (1GB) and now it's somewhere around 5GB free, but your uploads can only be 50MB max. Kinda sucks. I'm probably one of a small percentage of Mac people who used Skydrive but find iDisk a lot better. I tested Zumodrive on our iMac and it was not reliable at all; I was thinking of trying Dropbox but I'm happy w/ what I've got, and it's already 10GB (split w/ my Mail).
Truth be told MM is not a bad deal if you are using many of the functions (ie. iDisk, Mail, Sync, iPhone, etc.). I have a Yahoo account that is still my #2 (ebay, paypal, forums, craigslist, etc.) but use the MM account as my #1 for e-mail (ie. family, friends, colleagues, etc.). Were it not for aliasing, I would probably have gone to Gmail or something similar.