MobileMe, the company's replacement for the .Mac service, was expected to be rolled out Wednesday night after scheduled maintenance.
The service was activated Thursday morning, but users reported multiple issues with backing up data, as well as using the new Web portal Me.com.
"The MobileMe transition is underway, but is taking longer than expected," read a note on Apple's Web site Thursday afternoon. "Please refer to the official System Status to the right for current information on the availability of MobileMe services. We apologize for any inconvenience."
The company said the service is now fully functional, and its Web site now features a demonstration video of MobileMe's features.
Introduced at Apple's WWDC conference last month, MobileMe is a cloud storage service that will enable push e-mail and real-time synchronization of contacts, calendar items, and other information between multiple devices without a need for a Microsoft Exchange server.
"Think of MobileMe as Exchange for the rest of us," said Phil Shiller, Apple's senior VP of worldwide product marketing, during a presentation.
The service will work with the iPhone's calendar, contacts, and mail applications in addition to working with Mail, iCal, and Address Book on the Macintosh and Outlook on Windows. Users will be able manage their accounts through Me.com from any Web-connected device.
Customers can also use the service to store and share photos, videos, and other data wirelessly. The company said existing .Mac customers will be automatically transferred over. The service costs $99 per year and offers 20 GB of storage, with additional storage costing extra.