PHP 5.5 is finally available in our [extra] repository. The migration guide includes information about changes from version 5.4; a detailed list can be found in the PHP Changelog.
The APC package will be replaced by APCu. APCu only provides user data caching. For opcode caching use the new opcache extension. Alternatively we provide a development version of XCache which includes both, an opcode and user data cache.
Most applications will run without modifications, but watch out for deprecation warnings that fill up your log files.