MobileMe is a pretty awesome service. However, having used it for at least six months, I'm pretty well decided that it is not quite worth $100 a year. In addition, Google has just released, officially, it's support for CalDAV syncing between iCal and Google Calendar. Google's support of IMAP has also grown strong over this time period. Last, the final important piece of the puzzle is Dropbox, which is now open for public beta and works very well.
Setting up IMAP with google is simple and you can find directions here.
Setting up Google's iCal support is almost as easy, especially with their set-up tool. Directions can be found here.
The benefits of these combined services speak for themselves, but in any case, I'll repeat them here. Dropbox serves to replace the need for iDisk sharing since it works by synchronizing all of the files within given directories between all of your computers. Dropbox is not only useable on Mac OS X and Windows, it now works on Linux (Ubuntu only). CalDAV is also completely cross-platform using a product from Mozilla, Sunbird. Of course, given my love of OS X, CalDAV works extremely well within iCal on Leopard. Finally, email, being the most important of all, works on any email client that supports IMAP (almost every client). Not only this but, it magically syncs between all computers and my iPhone.
There are a few remaining features that I don't consider to be critical to the service because I don't use them or because there are other services that have existed for some time to handle the feature. Photo sharing, for example, is more than capably covered by Flickr. The greatest drawback, in my opinion, of this piecemeal approach is the lack of a unified website for when you are away from your computer. All-in-all, the MobileMe website is not worth the money and neither is MobileMe now, thanks to Google and Dropbox.