MobileMe always was a thorn in Apple's side. Their cloud syncing service isn't known for reliability, and with a $99 price tag, MobileMe never really took off with Mac and iOS users. Soon MobileMe is going dark, and being replaced by the sturdier and more iOS-friendly iCloud.
If you haven't moved to iCloud yet, now's the time. Apple will be flipping the switch on MobileMe for good on June 30, so you've got until the end of the month to transition your account, and find replacements for the MobileMe services that are being discontinued.
Moving to iCloud
MobileMe's core sync services aren't going anywhere, but you will have to transition to an iCloud account in order to continue syncing mail, contacts and calendars. To transition your account head over to www.me.com/move, and sign in with your MobileMe credentials. Follow the instructions to create your new iCloud account. Existing me.com or mac.com email addresses and aliases will continue to work.
But unlike email, not all of MobileMe's services are making the jump to iCloud. iDisk and Photo Galleries will be discontinued, so you should download copies of your data and pictures before the June 30 cutoff date.
On a Mac, you can connect to your iDisk in the Finder, once you're signed into your MobileMe account by heading to System Preferences > MobileMe. For PC users, the easiest method is to go to me.com/idisk in a web browser and sign into your account. Apple also has a support page with complete instructions for accessing your iDisk data.
To download your pictures from MobileMe's Photo Galleries, visit me.com/gallery in your web browser. Sign in, and click the link with your username at the top of the page. On the next page, click the Download icon for each album, which will save to your computer as .zip files.
Moving your data
There are plenty of options for replacing iDisk. Dropbox is a logical choice for cloud file storage, especially with its excellent iPad app. Box and SugarSync, also offer both free and paid storage options, and access via native iPad and Android apps. Google Drive is another good option, although there's no official iPad implementation yet (but plenty of legacy support for Google Docs through third-party apps).
For sharing photos online, Flickr and SmugMug (and plenty of other services) offer much greater flexibility than MobileMe's Galleries, and are supported by a wide variety of official and third-party apps. Photo sharing will also be more flexible with iOS 6. You'll be able to selectively share images from your PhotoStream, and friends who don't use iOS 6 or iCloud will have web access to shared images.
If you're using iWeb to build and host websites, you'll also need to move your files to a new host in order for your site to continue running. Web hosting packages can vary greatly in terms of price and services, but chances are, if you're still using iWeb, you can easily replace it with an affordable personal hosting plan for only a few dollars a month.
Despite the pain of transitioning to a new service, there is some silver lining to this iCloud. Documents in the Cloud simplifies working with an iPad by automatically syncing your files to multiple iOS devices, and there's a web interface for downloading files to your PC or Mac.
iCloud's calendar and contact syncing works just like MobileMe—and actually, we're finding it faster and more reliable. We experienced several catastrophic sync problems resulting in data loss under MobileMe, but iCloud has been syncing our devices flawlessly for months now.
If you do ever have a problem, iCloud's Backup comes to the rescue. Now, instead of having to back up to a computer, you can store your device backups in iCloud. Even more importantly, you can restore from those backups right on your device, no matter where you are.
For music fans who travel frequently, iCloud's optional iTunes Match add-on ($24.99/year) keeps your music collection up to date without having to manually sync with a computer. By matching songs in your iTunes library to tracks in the iTunes Store, you can listen to your entire library on the go, and download new tracks to your device on demand, without needing to manually sync to a computer.